Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part IV, Systems

Introduction

So far we’ve learned the basic components of the body, but how does it all work together? In a word: complicated. The horse’s body is a swirling mass of cellular activity, a complex series of systems that function together to give us the beauty of a living horse. And many of these systems are important for artwork because they dictate the possibilities of equine motion which we'll be discussing in greater depth through this series. However, understanding some other systems is handy for conceptualizing certain biomechanical functions subtly important for the artist as well. That's what we'll discuss here. Know it or not, these systems impact our creative decisions, some more subtly than others.

It should be mentioned, however, that the systems presented here aren't the whole enchilada. Only some have been chosen for discussion as an overview to inspire proactive research and because they more directly relate to how we sculpt. So these are just the bare bones systems regarding the equine body that make a great springboard into further research. Plus, for simplicity, the systems have been grouped in to eleven types: 

1. Nervous
2. Circulatory
3. Respiratory
4. Digestion
5. Lymph
6. Axial and Appendicular Systems
7. Passive Rebound
8. Shoulder Sling
9. Suspensory Apparatus
10. Reciprocal Apparatus
11. Stay Apparatus   

Nervous System

We’ve come to this sentence through actions which we don't think about. Scrolling down, processing the text, perhaps drinking some coffee while we read, maybe even deciding whether we should stop here and get some actual work done in the studio…every sensation and thought we’re experiencing right now, even those we’re not aware of such as the balance adjustments our bodies are making this very moment, the lingering taste of our coffee, our state of mind, and even unconsciously gauging the time of day by the sunlight streaming through our window....every sensation and perception we experience is accomplished by our nervous system. It’s the processing interface between ourselves and the world, and much of it occurs under our radar. That's to say the nervous system is the means by which the body communicates to the brain and the brain communicates back to the body in a continuous symphony of stimulation, interpretation, and action. 

Curiously, awareness of this system is sometimes ignored in equine sculpture because some artists tend to perceive a horse as a passive subject rather than an autonomous, feeling, thinking being with his own slant on reality, his own unique interface. Yet an awareness of his nervous system inspires interest in what he's experiencing  and that can do much to create convincing work. Quite literally, in order to sculpt our subject well, we have to crawl into his nervous system—his mind, spirit, and his body—if we want to capture an authentic moment.

As for how its constructed, the nervous system is made of nerve cells and fibers that conduct electrical impulses (messages) that interlace with every portion of the body, forming one whole networked system that communicates with itself. Nerves conduct messages to the brain (afferent) or from the brain (efferent) in a continuous stream of activity via the spinal cord. They’re based on two basic types of information: 

• Sensory information.
• Motor commands. 

The entire framework of the central nervous system includes the:

• Central and peripheral nervous systems.
• Endocrine (hormonal) system.
• Neuroendocrine (hormonal) system.
• Autonomic nervous system. 

An unconscious reaction to stimulus is called a "reflex" and such a pathway is often directly connected to the mechanism it affects such as the response of a tendon on its muscle body. A "plexus" is a grouping of nerves that usually are associated with a limb or body area such as the brachial plexus of the forelimb or the lumbosacral plexus of the hindlimb.

The central and peripheral system entails the nerves, spinal cord, and the brain. Specifically, the brain and spinal cord comprise the central nervous system and the outbranching nerves are the peripheral nervous system. Based on the body or environmental stimuli the nerves provide, the brain reacts to alter the body either with motion, emotion, chemical changes, or other such responses. 

The nerve cell (or neuron) makes up the basic unit of the central nervous system. These are gathered together to form bundles, ganglia, or fibers which are white or grey in color. On the surface, the nerve fibers usually lay in the protective valleys created by the muscles and other body features. The spinal cord runs down the length of the tunnel inside the vertebral column from the brain to the middle of the sacrum. The horse has a spinal cord that’s oval-shaped and about 79 inches long (200 cm). Between special holes in each vertebra, forty-two pairs of spinal cord nerves emerge. The cervical vertebrae have eight such nerves, the thorax has eighteen, the lumbar has six, the sacrum has five and the tail has five. The spinal nerves emerging from the neck and lumbar are larger because they’re associated with limb innervations. The phrenic nerve innervates the diaphragm, the mechanism that produces breathing by alternately creating a vacuum inside the torso. 

The brain, an extremely complex collection of nerves, weighs about a pound and a half (650 g) in a mature horse, about 1% of his bodyweight. Curiously, the equine brain is separated from the sinus cavity by membrane and not bone. Nevertheless, twelve nerves emerge from the brain, each responsible for sensory or motor stimuli. Simplistically, the horse’s brain consists of three principle parts, the hindbrain (rhombencephalon) which is comprised of the metencephalon and the myelencephalon, the midbrain (mesencephalon) and the forebrain (prosencephalon) which is comprised of the diencephalon and telencephalon. Granted, this discussion about the brain is supremely simplified, and the brain’s workings are far more intricate and elegant than these categorizations would make it appear.

The hindbrain sits by the brain stem and contains the medulla oblongata, the pons (a controller of emotions) and the cerebellum (a controller of motion). The hindbrain deals with attention focus and sleep autonomic functions, both reflex and complex motion and simple learning. The metencephalon, consisting of the pons and cerebellum, deals with arousal, fine muscle motion, tonus, sleep, balance, circulation and cardiac reflexes. The myelencephalon, consisting of the medulla oblongata, controls autonomic functions, breathing, digestion and swallowing, heartrate, and even sneezing.

The midbrain controls the sensations of scent and vision and some influence on the hindbrain’s functions, linking the hindbrain and forebrain. It also influences body motion and hearing as well. 

The forebrain is the largest part of the brain and made mostly of cerebral hemispheres. It establishes intelligence and personality, processes sensory stimuli and motor functions, it aids forethought and is involved with smell and touch sensations. It’s also linked to the pituitary and pineal glands, controlling several hormonal activities and reactions. In addition, it orchestrates sensory impulses throughout the body, maintains balance, controls vision, hearing, smell and taste while also controlling respiration, chewing and swallowing among many other functions. 

In more detail, the basic structure of the equine brain is as follows:

• Cortex: Deals with vision, vocalizations, hearing, memory and thought such as analyzing information and initiating complex responses.
• Olfactory Bulb: Deals with scent stimuli.
• Amygdala: Deals with arousal, autonomic responses from fear, emotional responses, and hormornal secretions.
• Thalamus: Organizes the brain’s sensory stimuli, except smell.
• Hypothalamus: Deals with autonomic systems, some emotional reactions, sexual behavior, endocrine functions, motor functions, the sleep/wake cycle and regulates food/water intake. 
• Pituitary Gland: Produces important hormones, regulates other hormone producers, and regulates the endocrine system.
• Pineal Gland: Creates sex hormones, regulates endocrine signals and functions, and creates the feeling of sleepiness. 
• Cerebellum: Maintains balance and coordinates complex motion. 
• Pons: A processor of emotion, helps with autonomic functions and helps the Cerebellum.
• Medulla Oblongata: Deals with involuntary functions.

The size of the equine brain is comparable to our own. The cerebellum in the horse is proportionally about nine times larger than in humans which makes sense for coordinating the movements of four legs. The horse’s olfactory bulb is also larger than that of humans, indicating that the horse’s sense of smell is important. Also interesting is that the two lobes of the olfactory bulb are directly connected to his nostrils; each lobe is connected to a nostril. He has a well-developed cerebral cortex that’s densely wrinkled, like ours, even though his brain weights less than ours. The interesting comparison is with a dog, who lacks the many wrinkles in horse or human, yet we all know how trainable is a dog. This suggests the horse is far more intelligent than many give him credit for. The head and face of the horse is also well represented in the cortex whereas in humans, the hands have this dominance. Overall, horses are cognitive animals possessing a superb ability to learn, interpret, and respond to stimuli. They aren't stupid or dull-witted, by any means. Of course, they would have to be...dealing with us .

The autonomic nervous system can be divided into three parts, the sympathetic, parasympathetic, and the enteric, and has to do primarily with connecting the innards with the brain. However, an exciting new “rediscovery” has brought this entire system under fresh scrutiny. The study of the independent nervous system in the gut dates back to about the 19^th century with the English scientists, William M. Bayliss and Ernest H. Starling who performed experiments on the intestines of dogs. The German neurologist, Leopold Auerbach is also credited with pioneering and actually documenting research in this area. They were the first to discover that the gut operated on an independent level from the brain and, in essence, alluded to the enteric system, or the "Second Brain" contained in the gut. In fact, some refer to this as the "First Brain" since digestion is a basic component of life, even without a conventional brain. Sadly, however, attention waned regarding this system and it became dismissed as a mere collection of relay ganglia. 

However, some new studies conducted by Dr. Michael Gershorn, of the University of Colombia in New York, have rediscovered the complex influences of this Second Brain. Through his studies, the enteric system is being recognized as a beautifully intricate system that’s so incredibly brain-like, both in structure and function, as to have a similarly complicated network of microcircuitry based on neurotransmitters and neuromodulators, components also found in the central and peripheral nervous system. In fact so brain-like is the enteric system that it's also vulnerable to Parkinson's disease and Alzheimer's disease as seen in humans. 

In addition, the Second Brain is comprised of what’s believed to be about 100 billion nerve cells, more than the spinal cord contains. Ultimately, this allows the enteric system to operate independently of the central nervous system, or brain. It also has a network of neurons connecting to other organs such as the pancreas and gallbladder, as seen in humans. The vagus nerve is the primary means of message exchange between the enteric system and the central nervous system and, indeed, their communication is so intimate that it’s often difficult to discern “who’s saying what to who and when."

Furthermore, the Second Brain is responsible for the production and storage of the third neurotransmitter, seratonin. Previously, norepinephrine and acetylcholine were believed to be the respective sympathetic and parasympathetic transmitters, creating a tidy, "two neurotransmitters, two pathways" theory. This has since proven insufficient and it’s now believed at least thirty chemicals instruct the brain and, in fact, all these chemicals are also present in the enteric system. 

Although more research is necessary, there seems to be a link between mood, thought, reaction, and the enteric system. It’s also hypothesized that the Second Brain may store information from previous physical and mental stimuli to influence later reactions. It's also suggested that it may create the feelings of “butterflies," “instinctual gut feelings," the physical pain of grief, the floating elation of joy, and other emotional states that entail physical sensations. Some have even gone so far as to propose that the enteric system may be making unconscious decisions that are later taken credit for by the brain. Indeed, certain studies have shown that even the raising of an arm has registered its activity in the brain about half a second later after the actual action, implying that another mechanism might be involved. The influences of the enteric system also has consequences for physicians who prescribe mood-altering medications. The common side effects of such drugs are diarrhea or constipation, which are certainly digestive issues. 

Nevertheless, there are differences in the chemical communications of the nerve cells in the enteric system between species, so conclusions made of one animal can’t necessarily be applied to another. Regardless, the research being conducted with this “new” system will result in more fascinating information to be sure. In any case, in the horse, the Second Brain is enormous, about the size of an opened umbrella that networks itself throughout the entire digestive tract.

Anyway, the endocrine system entails the hormones, the chemical messengers in the body. Produced by glands and carried in the bloodstream, they control organs, reproduction, metabolism, growth, and other body functions, and even influences emotions. While the basic unit of the nervous system is the nerve cell, the equivalent in the endocrine system is the secretory cell, the cell responsible for creating the hormones in the hormonal glands.

The neuroendocrine system is a combination of both the nerves and the hormonal system, linking the environmental stimulus to the internal stimulus. This explains how the length of daylight may be a timer for shedding, mares coming into season, etc. Within this system, however, nerves can also make hormones as stimulus would dictate. One example is the nerve-created oxytocin stimulation, prompted by the placenta and fetus, to the mare’s body during birth to drop the milk into the udder, otherwise known as the “milk let-down reflex." The neuroendocrine system may also be responsible for creating peptide hormones, which among them include endorphins.

Overall, the nervous system is not usually seen on the surface topography of the horse, except sometimes for three branches on the massater, beneath the teardrop bone. However, artists can sometimes inadvertently recreate nerve disorders by not understanding symptoms of nerve damage or degeneration such as atrophied shoulder muscles or "knuckle-dragging" hooves. Yet the important thing to remember is that the horse is emotion in motion; his emotions govern his thinking mind and his moving body. He's emotion in physical form. That means it’s prejudicial to credit his reactions as merely behavioral and instinctive. The equine is so much more than instinct! This is important to remember when interpreting and depicting his movements and expressions, things relayed by his nervous system. 

Circulation

In the horse, blood is circulated by the heart through arteries, veins, and capillaries at about 6.6-10.6 gallons (25-40 liters) per minute. Veins move blood towards the heart and arteries move blood away from the heart (except with the aorta and pulmonary vein where this is reversed). The heart of an average 1,000 lbs (373.25 kg) horse weighs about 10 lbs (3.73 kg) and is about the size of a large melon. The endocardium is a smooth membrane lining the chambers that helps to reduce friction between the chamber walls and the blood cells. The myocardium lines the heart muscle. The pericardium, similar in structure to the endocardium, helps to reduce the friction between a pumping heart and the surrounding structures in the chest cavity.

The equine heart has four chambers, the left atrium, left ventricle, right atrium and the right ventricle. The left side is separated from the right side. This makes the left atrium and ventricle operate as a team to pump oxygen-depleted blood (from the pulmonary veins) through the aorta and into the lungs where the blood gathers oxygen. In turn, the right atrium and right ventricle, operating as a team, accept the oxygen-rich blood from the lungs (from the vena cava) and pump it back into the body via the pulmonary artery. The walls of the atria are thinner than those of the ventricles since the latter has more work to do in terms of pumping power. The right ventricle actually has the hardest job of the four by having to pump the blood back into the body with sufficient force to propel recirculation, so its walls are quite thick and powerful. Working in concert, the heart beats in a rhythmic fashion, synchronized to efficiently maintain circulation. The contraction starts in the first chamber, but each contraction only affects one part of the heart at a time, allowing the portion immediately behind to relax to permit more blood to enter. So contraction (systole) is followed by relaxation (diastole); each wave of contraction is followed by a wave or relaxation.

However, this wouldn’t be possible if it weren’t for the valves that prevent backflow of blood. The left side has the mitral valve (two-cusp valve) and the right side has the tricuspid valve (three-cusp valve). Valves also exist in the aorta and pulmonary artery. The aorta and the pulmonary artery have walls containing a lot of elastin, too, allowing them to contract and expand. As they do so with blood flow, their forces oppose those imposed by the pumping heart. So when both pressures equalize on either side of the valves, they shut to allow the pump to continue as the first chamber again accepts more blood. In short, the valves force the blood to flow only in one direction—forwards. The rhythmic pumping action is produced by two special conditions of the heart. First, heart muscle fiber is formed into sheaths rather than individual fibers grouped together into bundles. The heart muscle is also unique in its intrinsic rhythmic contractions. Most of all, the heart has a special conducting pathway that stimulates the muscle sheaths in sequence, creating the wave of contraction. This pathway originates on the right atrium as the sinus node which activates the atrio-ventricular node (AV node) which then passes through to the ventricles via conducting fibers known as the Bundle of Hiss, and from there spreads to both of the second chambers. It’s this electrical stimulation that initiates contraction that can be recorded on an electrocardiograph (ECG).

Because of all this, the pumping heart can be heard as a “lubb-dup” sound; the “lubb” is a slow sound and the “dup” is a shorter, more pronounced sound. These are referred to as the first and second sounds of the heartbeat. The first sound is created by the atria as they contract while the second is produced by the snappy shutting of the valves in the aorta and pulmonary artery. The normal heart rate for a horse is forty beats per minute and for a young foal, eighty beats per minute. Predictably, the heart rate increases with exertion. 

Blood is pumped through the arteries, which branch into smaller vessels as they feed all the parts of the body. When the vessels become quite small, they’re referred to as arterioles (small arteries) which then branch into even smaller capillaries, whose walls are only one cell thick and permit diffusion of oxygen and wastes through their membranes. These arterial capillaries then become venous capillaries and the whole system is mirrored in ascending size back to the heart. Blood returning to the heart from the hindend do so in the posterior vena cava and that returning from the frontend do so in the anterior vena cava. The walls of these structures contain elastin, smoothing the pressures of pumping forces and helping to protect the tiny capillaries, and also creating the sensation of pulse. The decreasing size of the vessels into arterioles and capillaries also helps the pumping mechanism by providing resistance since allowing pressure to rise creates more force for pumping throughout the body. Chronic high blood pressure is unknown in horses, except during exertion or sickness, which when these pass, so does the high blood pressure. The heart has its own circulation system within its myocardium that siphons off blood fresh from the lungs, providing itself with the most oxygenated and freshest blood first.

In hot weather, stress, exertion, and sometimes sickness, the veins and capillaries can become more prominent as the blood flow is increased, especially on thin-skinned breeds. Indeed, the capillaries may become quite obvious and help along the idea of our narrative. It's also thought that the veins and capillaries help to diffuse heat under such conditions.

Respiration

The respiratory system of the horse is a complex design. In sequence, the air passes through the nostrils into the nasal cavities (and past the hard palate) and past the turbinates to the sinus cavities where it passes the Eustachian tubes (each with their Guttural Pouch, a structure unique to horses) to the pharynx (back of the throat), past the soft palate, to the larynx (voice box) then onto the trachea (windpipe), through the bronchi, and down to the lungs where the diaphragm and ribs create inspiration, then back again during expiration. In the lungs, oxygen is exchanged for carbon dioxide and other wastes produced by the body, which are expelled during expiration. This (and the Palantal Drape, or soft palate, which we'll discuss later) makes the horse an obligate nose breather, i.e. he cannot breath through his mouth.

Coordination of the entire system is more complicated than one would think, too, since certain parts have to physically morph to accommodate either breathing or swallowing. And to make things a bit more complicated, air, food, and water have to be channelled properly through the open space of the pharynx and by the seven openings within it! Those openings are a return to the mouth, two internal nares, (where the nasal cavity empties the air into the pharynx), two Eustachain tubes, an esophagus (the food tube) and then the larynx (the air tube to the lungs).

The true nostrils (or nares) are direct openings to the nasal cavity. Their front, medial rims are made of stiff cartilage to prevent them from collapsing shut during inspiration. An upper pocket above them forms the false nostril, which functions to filter off and collect dirt and debris as it enters the airway. 

The nasal passages are divided into two bilateral halves by a middle nasal septum and each are bordered by the nasal bones, maxillary, and hard palate. At the back of the nasal cavity are three delicate scroll-like bones called turbinates, covered by thick mucus membranes. The first two are the dorsal (upper) and ventral (lower) turbinates that provide a broad surface area for discriminating scent. The third, the ethmoturbinate, is rich in olfactory nerves and transmits the scent stimuli to the brain. All three are also rich in nerve and blood supplies while also thickly covered with mucus and fluid-producing glands that moisten air and filter out particles. These three chonchae further divide the nasal passage into three channels, the dorsal meatus, middle meatus, and ventral meatus (the largest of the three and a direct pathway from the nostrils to the pharynx) which are believed to channel airflow directly to the olfactory nerves. 

The nasal cavity also has a series of paranasal sinus cavities, on each side of the skull, that help to add moisture, warmth, and filtering capacity to the incoming air but play no part in the ability to smell. These cavities are the rostral maxillary, caudal maxillary, frontal sinuses plus sphenopalatine and ethmoidal spaces. What are sinus cavities? Welp...they're hollows in the skull. As the horse grew in size during evolution, his head had to increase to house the battery of teeth he requires as a grazing animal. However, had his head grown in bony substance alone, it would be of a prohibitory weight indeed. The sinus hollows permit a large skull sturdy enough for the job of grinding food, but removes excess weight in turn. 

The Eustachian tubes foster pressure equalization between the inside and outside of the head to maintain a constant head pressure. “Ear popping” is a common manual use of the Eustachian tubes, something we often do on a plane trip. They run from the inside the head to the ears (or bulla in horses, which holds the ear). However, during major exertion (like galloping), the wind streaming through the horse’s nasal passages comes in at the tremendous speed of 400 mph (644 km). This is easy to put into perspective when one realizes that no wind exists on the planet that fast. The current records are 280 mph (451 km) in Antartica and 318 mph (512 km) in an F5 tornado. The vacuum created by this intake of air would quickly burst the horse’s eardrum, among other things, without mediation. The Eustachian tubes serve this purpose by equalizing head pressure. But they’re also helped by the Guttural Pouches (which lay outside the pharynx) with their two functions. First, the pouches are made of epithelial tissue with a hefty immune system (so much so it can even be considered a gland of the immune system). And, second, the pouches prevent a dangerous vacuum inside the head by acting as an “eddying pouch” for air. Each pouch can hold about 10 oz - 17 oz (300 ml - 500 ml) of air. No wonder why they're unique to horses, a large animal dependent on running at speed for a long time to survive!

Now additionally, the speed of this incoming and outgoing air is of particular relevance to Arabians, specifically to those with "deep dishes" or "exotic," extreme, "typey" heads. When the head is of this configuration, it angles the sinus cavity away from what nature intended and into a series of improper bends. As a result, that rushing of air at 400mph hits the delicate sinus tissues at unnatural angles, causing inflation, pain, and bleeding. Such horses also have a hard time breathing because of this, not being able to intake enough air, and the unnatural angles also cause them to wheeze or make "gruntly" sounds when made to do anything significant, something that's become more prevalent in Arabian "halter" classes. For these reasons, the "exotic" head on an Arabian is actually off-type because it prohibits the individual from being anything more than a "lawn ornament," something decidedly antagonistic to the Arabian's renown soundness, functionality, and endurance. So is this something we want to validate in our art work?

Anyway, the pharynx is a single muscular cavity separated from the back of the mouth by the soft palate or Palantal Drape (discussed below). In a mature horse, it’s about 5.9 inches (15 cm) long. It’s upper portion is sometimes called the nasopharynx and is where the Eustachian tubes exist. 

The larynx, which joins the pharynx and trachea, is a short tubular structure made up of five cartilages that form a rigid framework that articulate together. One of these cartilages is the epiglottis which folds back during swallowing to protect the lungs. It performs three functions:

• It regulates the volume of air during respiration with its contractile abilities.
• It prevents aspiration of food into the lungs.
• It contains the voice box, providing vocal communication. 

Adequate intake of air is critical for a cursorial animal and, in the horse, nature has designed the internal nares to be targeted directly over the larynx to literally inject air into the lungs. The horse has turbo injection!

The trachea is a fibroelastic tube attaching the larynx to the bronchi of the lungs. The “C” shaped rings of hyaline cartilage (which are incomplete dorsally) that keep it permanently open also make it resemble the hose of a vacuum cleaner. It’s palpable on the lower surface of the neck and is about 30-31 (75-80 cm) in length. 

The bronchi are the right and left branches of the trachea that channel directly into the lungs. They reduce in size into bronchioles until they approach the alveoli of the lungs. The right and left lungs form large triangular thin spongy sheets that occupy most of the thoracic cavity. They’re covered by a series of membranes, called pleural membranes that facilitate friction reduction as they expand and contract with respiration. 

The diaphragm attaches to the 11^th or 12^th rib and is therefore oriented on a forward tilt to follow the tilt of the ribs. It forms a cone or “umbrella” in the torso with its top attaching to the bottom of the spinal column at the last of the thoracic vertebrae. To inhale, the diaphragm contracts backwards, making its cone shape more pronounced, creating a vacuum inside the chest that sucks air into the lungs. This can also be accomplished, but less effectively so, with the lifting of the ribs. This is similar to the difference between “deep breathing” in the gut and “shallow breathing” in the chest applicable also to opera singers. Each rib has two swivel joints, allowing it to have a forward roll and also an up and down roll with its vertebra. Only the first eight ribs are directly connected to the sternum, but loosely so with cartilage, so they really only abut the sternum. The last ten ribs aren’t connected, but are tied to the rib preceding it and onto to the first eight ribs via a bridge of cartilage linking them all together along the bottom. Thusly the ribs can roll with breathing, creating that familiar rolling rise and fall of the ribcage quite noticeable after a good workout.

During exertion, the horse dilates his nostrils, nasopharynx, and larynx to intake air. What’s interesting is that the motion of his body, particularly at the gallop, is synchronized with breathing. Specifically, during the suspension phase of the gallop, when his head is up and his gut is shifted backwards is when he’ll inhale, then during the extension phase, when his head is down and his gut is shifted forwards, is when he’ll exhale. The more rapid his strides, the more rapid his breathing is automatically. 

Anyway, the massive decrease in air pressure in his airways during inhalation would make the larynx collapse it weren’t for a special muscle (the dorsal cricoarytenoid muscle) to dilate it, which also increases airflow as well. Contraction of this muscle also pulls the vocal cords out of the air stream. In fact, active muscle contraction is required to keep the vocal cords contracted and alongside the walls of the larynx, out of the path of the airflow, during exertion. However, “roaring” is a paralysis of the left vocal cord due to trauma of the left laryngeal nerve that controls it. This makes the left vocal cord obstruct the airway, effectively creating an obstacle for airflow and respiration. Roaring can also be caused by localized infections, certain toxins, or other things that would damage the nerve. However, many cases of roaring are caused by trauma to the left laryngeal nerve which governs the left vocal cord. This branch is especially vulnerable because it wraps around the aorta, unlike the right branch. Therefore, it has less stretching ability or play than its right branch. So when the right forehoof is planted and the head is rapidly jerked to the right and back, this can stretch the left branch of the laryngeal nerve, injuring it and potentially paralyzing the left vocal cord. However, an emotional form of roaring can happen as well when the horse gets “choked up," tightening his laryngeal muscles, because he’s upset. This causes a gruntly noise, a common occurrence in some show ring classes today. 

The effects of breathing aren't often expressed in sculpture. For example, we often see running pieces with "normal" ribcages when, in fact, they should depict the necessary changes we'd see during hard breathing. Likewise, with whinnying pieces, we typically don't see the natural exertion of the ribcage and abdominal muscles that would be required to push air out of the lungs to vocalize. If we observe a whinnying horse, too, we see that each "puff" of a whinny in its later phase is accompanied by a push of the ribcage and abs, causing his belly and torso to "chatter" in synch, something we often don't see expressed in sculpture either.

Digestion

The horse’s intestinal track exists in this sequence: mouth, esophagus, stomach, small intestine (divided into the duodenum, jejunum, and the ileum), large intestine (divided into the cecum, great colon, small colon, and rectum) and anus. The great colon also has four parts, the right ventral colon, the left ventral colon, left dorsal colon, and the right dorsal colon. In the small colon is where those “road apples” are made as it absorbs the excess water from the material, leaving firm balls of waste matter. The small intestine is about 70 ft long (21 m) and can hold about 11-13 gallons (40-50 liters). The large intestine is about 25 ft (7.62 m) long. Its cecum can hold about 6-8 gallons (25-30 liters) of fluid. Its great colon is about 10-12 feet (3-4 m) long with a holding capacity of about 13-16 gallons (50-60 liters) while its small colon is about 10-12 feet long (3-3.7 m).

A horse will typically eat about 2%-2.5% of their body weight in dry matter a day and maybe up to 3.3% if on pasture 24 hours a day. That’s at least about 20 lbs (7.5 kg) of food daily for a 1000 lbs (373.25 kg) horse. The horse has an curious digestive system, a combination of being a ruminant and non-ruminant. It’s actually a rather archaic system.

Non-ruminants (such as people, dogs, pigs) use the actions of enzymes to digest the proteins, carbohydrates, and fats from food. In contrast, ruminants (such as cows, deer, and sheep) make use of bacteria to ferment and break down fibers in their stomachs then use enzymes in the small intestines to digest the food. However, the horse is a strange blend of the two systems (more on this later in the Evolution portion of this series). He has his actual digestion with enzymes in the fore gut before the cecum. About 52-58% of the crude protein digestion and nearly all soluble carbohydrate digestion (except fiber) happen here. Then the materials move to the cecum, which is enormous equivalent to the human appendix and contains colonies of symbiotic bacteria, and then the colon where other microbes take over to break down the fiber. Only bacteria can breakdown the glassy cellulose skeletons of plants, no mammal can, so this symbiotic relationship allows the bacteria to use what they need, then they leave the rest for the horse to use; fermentation here creates large amounts of volatile fatty acids produced for absorption. The plus here is that this design allows him to process simple carbohydrates in the fore gut then digest their fibers in the hind gut. However, the downside is that this system is less efficient than a ruminant and also prone to digestive problems such as blockages (which can cause colic). If digestion took place in the stomach, a lot of the horse’s digestive issues could be avoided.

The horse’s stomach is really quite small for his size, only able to hold about 2 gallons (7.57 liters) of material. This is because digestion doesn’t really occur there, but rather, its purpose is to create a slurry with digestive fluids and to further mash up the materials. The horse must also consume large amounts of water to create the “slurry” required by his gut for proper digestion (in the wild, much of this moisture is provided by the un-dried plants themselves). So in order to accommodate the volumes of food and water he must ingest, the stomach empties about every twelve minutes and the materials progress down the small intestine at about one foot per minute. As a result, the total time from mouth to cecum for a lump of food is about one and one half hours (However, pelleted food is moved through the system faster than normal hay.) The large intestine of a mature horse comprises more than half of the total digestive track because it’s crucial for bacterial fermentation of food and storing water. Overall, it takes about two to three days for ingested food to be turned into manure. The smallness of his stomach and the rapid transport of digesting food are other reasons why he must eat almost continuously.

In contrast, a foal or immature horse doesn't have a developed cecum or colon for bacteria-aided digestion yet. In fact, there’s very little of this type of digestion before he’s three months old. Consequently, a foal should have a low fiber diet, which is, of course, provided by his dam’s milk. Foals observed nibbling on their mother’s manure are believed to be seeking the bacterial culture needed by their gut for digestion. 

The gut is suspended from the spine by the mesentery, a membranous sheet that cradles the intestines. It’s narrow at the top, bunched into pleats, and flares out at the bottom as it suspends the viscera. But this does make it vulnerable to twists that can cut off the blood supply, killing off that portion of the intestine and mesentery, as can be implicated in colic. Sometimes a hole will occur in the mesentery, called a "tiflum," through which the intestines can get flipped into to create a similar situation. A tiflum can be inherited in some bloodlines or breeds.

In regards to swallowing, the horse again has a system that’s vulnerable, particularly to human interference. Inside the skull are three bones for swallowing, together called the hyoid apparatus, or "swallowing sling." These bones are the:

• Basihyoid.
• Keratohyoid
• Stylohyoid. 

All three support the walls of the larynx. (Hyoids are remnants of the old gill bar in fish.) These bones slide back and forth and up and down during swallowing, producing that "glug glug" motion. In humans, the hyoids are unattached and “float," but in horses, they’re attached via muscle and ligaments to the tongue, ear, and humerus. This means that yanking on the tongue or ear twitching can injure the hyoids and their supporting tissues. In turn, this may cause the entire swallowing apparatus to become injured and painful, or make the larynx reseat crookedly, or even lock the entire mechanism. Then how will the horse swallow? 

And as for the humerus connection...when the horse lost his clavicle, this forced the Omohyoideus muscle to look for a new attachment to stabilize the hyoids, finally connecting to the humerus. So in the modern horse, the Omohyoideus comes off the hyoid bones, becoming part of the Brachiocephalicus muscle that connects to the arm. Through this mechanism, the delicate hyoid bones can be jerked by the motions of the neck and foreleg. This usually isn’t a concern with an unridden horse, who naturally protects his hyoids, but human influence can sometimes override the horse’s ability. For instance when being ridden, the hyoids can be jerked when a foot is planted and fixed and the neck is cranked, hard and rapid, in the opposite direction. In addition, horses can’t swallow with an open mouth (and neither can we) which means that irresponsible practices that force a horse to hold his mouth open for long periods of time inhibits his natural habit of swallowing periodically, which can lead to stress, discomfort, and excessive drooling.

Now when a horse swallows, the walls of his pharynx contract to push the food into the esophagus, but this food still has to cross the open gap of the pharynx. A concern then arises because the esophagus (food tube) is situated above the larynx (air tube) creating a risk that food bits might inevitably be pushed or sucked into the lungs. But there exists a special little mechanism that prevents this, the epiglottis, which covers the aditus larynges, the opening of the larynx and voice box, to protect the lungs from food or water. While swallowing, muscle contraction and the actions of the swallowing mechanism itself automatically smoosh the epiglottis closed, and then the horse swallows. But why is the food tube above the lung tube in the first place? Shouldn't it be the other way around? It's fish who have the same design since they breath with their gills and eat with their mouths. It’s believed in early evolution, when fish came onto land, this orientation didn’t change in the descendants of fish. 

Horses have a unique large, soft palette, the Palantal Drape, which is a muscular, tough, stretchy sheet of flesh that blocks the mouth from the pharynx. It lies at the front of the pharynx and its lower ends are tacked down near the larynx. It’s equivalent to the human uvula (the little “punching bag” at the back of our throat) but of enormous size and specialized design in the horse. Except for one-way slits in the middle and along the sides through which food and water are squeezed, it forms an effective wall. In fact, upon inspection of the back of the horse’s mouth, one doesn’t see down into his throat, like with us, but a pink sheet of flesh...that's the Palantal Drape. It means the horse can’t breathe through his mouth, but it also means he can't vomit. Because of the design of his throat, vomited food would be forced through the larynx down into the lungs and through the nose. Either way, the horse would be in pretty serious trouble. Therefore, nature has provided the horse with a one-way valve at the top of his stomach to prevent vomiting early under normal circumstances. 

Now regarding the horse’s hard palate, one notices that it’s ridged and somewhat vaulted. These ridges help the tongue to push food back to the grinders and to the back of the throat for swallowing. Now the interesting thing about hard palates is in what critters they occur. Most reptiles lack hard palettes or differentiated teeth since their modus operandi for eating is to “snatch and gulp,” with digestion occurring entirely in the gut. In contrast, mammals (particularly herbivores), “bite and chew," and thereby require incisors, canines, and a host of grinding molars. Also, digestion begins in the mouth with the grinding of the food and mixture of saliva, necessitating a hard palate to provide a firm surface for that to happen. But this allows mammals to better maintain their warm-bloodedness, which requires more energy than being cold-blooded. Also the hard palette divides the food flow from the air flow, allowing mammals to breath while eating, allowing a more continuous and rapid intake of food. On the other hand, most reptiles and amphibians have to hold their breath while they eat since they lack a hard palette. So in short, a hard palette helps the body to fuel its warm-bloodedness better.

Horses chew in a circular motion, sometimes having a favorite side. (This side also usually corresponds to which side the horse favors in motion, or leans to if he's a crooked mover.) The basic muscles for chewing are the Masseter, Temporalis, Buccinator, and Taragoideus. Chewing muscles must be symmetrically developed, or tooth problems are possible. The Masseter pulls the jawbone up and into a sideways pull. The muscle has two layers at opposing angles, both being full of septa (gristle fascia). The rear third is attached directly to the bone via the periosteum. The Temporalis pulls the jaw tightly closed, putting pressure on the back teeth for grinding. This Temporalis muscle in Quarter Horses is often large, often referred to as “worry bumps” or “wisdom bumps." The Taragoideus (on the inside of the jawbone, opposing the Masseter from the inside) works in concert with the Masseter, creating the typical rotational chewing motion of equines. Finally, the Buccinator (buck-sin-ator) aids the tongue by orientating the food onto the molars, and helps to crunch the top and bottom molars together. The chewing action causes the coronoid process of the mandible to pop in and out of the temporal fossa (the hollow above the eye orbit, or "salt cellar") that's easily observed when the horse is chewing. 

The tongue is the largest muscle in the head and plays in important part in chewing and swallowing. The hyoids have many muscles operating on it, but simply put, help the Omohyoideus muscle in the swallowing action. The Digastricus muscle joins the Styloidhyoideus muscle to form the Digastricus sling, an important mechanism for swallowing and lifting of the larynx. Many of the other facial muscles are primarily for breathing or expression rather than chewing (which we’ll discuss later).

Many sculptures of horses with open mouths too often lack the Palantal Drape, or in other words, they have our eye able to go right to the back of the throat as though the Drape didn't exist at all. 

Lymph

This system is designed to maintain the balance of fluids in the body, especially in the legs so they don’t become swollen with fluids. It has a circulatory system similar to that of the heart, but it depends on the massage of the moving muscles and tissues for its pumping action. Valves within its tubes ensure that the flow keeps going forwards to the heart, where it drains into the blood flow close to the heart. The lymph walls are very fine, allowing excess liquids to diffuse into its system and be recirculated. Swelling or edema (filling) indicates that the free flow of water and liquids is meeting resistance in the lymph system.

The lymph system also contains lymphocytes and some other white cells involved in the immune system. Being so, the lymph system is an important factor in healing from injuries because it removes excess fluids and toxins caused by swelling or infection. Indeed, at different points along its circulatory system are glands (lymph glands) that skim off infections or toxins. The glands also help to synthesize antibodies based on what it filters from the bloodstream and interprets as a threat. Infections can swell the glands, as is sometimes the case with those seated between the bars of the jaw, such as with strangles.

Most equine sculpture have faulty lymph systems, as indicated by swollen or puffy, indistinct legs and leg joints. Healthy leg joints aren't smooth and rounded, but have distinctive, crisp surface topography created by the underlying bones, ligaments, and tendons.

Axial and Appendicular Systems

The equine skeleton is comprised of three overall systems, as follows:

• Axial Intrinsics: Muscles that only touch the bones that deal with the axial body. For example, the Masseter and Multifidis. Note: The hyoid system should be included here.
• Appendicular Intrinsics: Muscles that only touch the torso, having nothing to do with protraction or retraction of the limbs. For instance, the Supraspinatus or Infraspinatus. Note: We can think of the pelvis as part of Appendicular system when it comes to leg function.
• Bridging Muscles: Those muscles that connect the Axial and Appendicular systems together to marry the whole body together. In essence, they're necessary for motion. Being so, they also happen to be more prone to injury, and bad riding can compound the stress they undergo, which is why body therapy tends to target these muscles.

It's important to understand that Axial muscles dictate Appendicular muscles, as well as the mouth. And while the Axial and Appendicular muscles can operate independently, it's the Bridging muscles that connect all the functions to make motion possible.

Passive Rebound Ligamentary System

There’s a mechanism in the horse often overlooked, particularly in riding, that’s the secret for all good movement. This system involves the ligaments of the neck, back, and pelvis, and work on the spine. This is a passive system, relying on the stretching of these ligaments rather than depending on muscle contraction to hold the posture. This system also ties into the horse’s unique Stay Apparatus (discussed below).

A biological truth about the horse is that his spinal dynamics always dictate his leg and mouth dynamics. Now the mouth may seem like a strange connection, but his mouth is greatly influenced by how the spine is used. The skull’s hyoid bones, the function of his mandible joint, and even his chewing “handedness” are all linked to his spine and his body “handedness." In other words, the horse uses his legs and mouth according to how he uses his spine. In fact, problems in the mouth can inhibit the use of his spine properly. In every way, in regards to his biomechanics, the horse is ruled by his spine. 

Now the real structure of his spine is hidden by its outward appearance, yet it’s extremely important for us to understand its true nature. For starters, the dorsal processes of his spine make his back appear hollow when, in fact, the spinal column itself is arched upwards (despite what many erroneous charts and mounts depict). This arch helps to support his heavy gut with minimal muscular effort. It also means the dorsal processes of his spine never touch under natural conditions. A helpful metaphor for the structure of his back is a canterlever suspension bridge such as the Golden Gate Bridge. His withers and LS-joint are the pylons and the connecting linkages of his back comprise the suspension portion. The entire structure is linked and suspended by the Passive Rebound System. But what makes this system? And what does it do?

The Nuchal ligmament in the neck, both the funicular and lamellar portions, blend into the Supraspinatus ligament, effectively linking the poll all the way down to the sacrum and onto the tail. The Sacro-sciatic ligament of the pelvis also connects to the Supraspinatus which then, in turn, blends into the dorsal fascia. The whole system is so interconnected in this region that, in fact, manually lifting the tail and making a “window-washing” motion alternately pulls on either “roof” of the the Sacro-sciatic ligament (which creates a handy relaxation therapy for a stiff back). In turn, the sciatic notch of his femur attaches to the Sacro-sciatic ligament, linking his hindleg to this entire system. In total, this entire set of linkages is called the Passive Rebound System.

Evolution developed this system to minimize muscular effort (i.e. energy costs) for him to hold his posture and to also facilitate the fight-or-flight response. By being stretched, the horse can rely entirely on the ligaments of this system to support his head and neck and to stabilize his back. Because this system is linked to the Stay Apparatus of the hindlimb (which we'll discuss in a bit), it also allows him to rest standing up with minimum energy. 

Now in terms of the fight-or-flight response, the Passive Rebound System stores energy as the belly muscles are contracted, which stretch the ligaments like strings on a bow. When this energy is released by the relaxation of the belly muscles, the energy stored in this system is released, aiding the Longissimus dorsi to leap the body forwards. This is why the horse arches his back and coils his loins when he’s about to take off, typically raising his tail, like a white-tailed deer, as the Passive Rebound System clicks into gear. This tail raising effect also is an alert signal to horses, which contributes to why horse herds seem to move off as a flock in a flurry of motion.

As he takes off then, the energy stored in the Passive Rebound System releases like a spring, helping to produce that powerful leap forward into a gallop to escape a threat. This system is so efficient that very few predators can catch a horse once he has leaped forward into a gallop. Indeed, the only animal that can best do so is another horse.

However, all this changes the moment a rider gets on his back. The typical reaction a horse has to a rider is to hollow his spine, tensing his back muscles and bracing his neck. This posture injures the horse in many ways, and is psychologically traumatic as well, since it's painful to him. Horses ridden thusly then tire and injure quickly since this posture is so awkward, unnatural, and inefficient. The point of proper riding then—of riding in bascule—is to reinstate this natural arc to the spine and compel the animal to again rely on his Passive Rebound System to carry both himself and the rider. In doing so, his gaits gain kinetic energy, making them easier, agile, enduring, and more elastic, and free from riding-induced injuries.

Many sculptures demonstrate a misunderstanding of this system such as we see in leaping horses with hollow backs and a level pelvis, or moving horses that don't demonstrate the natural coordination of this system. Indeed, the torso, i.e. spine, is often static in sculpture, as though it was simply a circumstantial connection between the forequarter and hindquarter.

Suspensory System

This system is comprised primarily of the suspensory ligament along with the foot and sesamoidean ligaments.

The suspensory ligament is a yellow ligament (i.e. really an ancient muscle), tough, and rubbery enough to help support the leg (specifically the pastern joint) by opposing the forces of gravity and impact. It begins within the carpal tunnel, attaching to the inner backside of the carpal bones. It then runs down the leg, next to the bone, to split into two branches, each running over a sesamoid bone, bending forward and across the 1^st and 2^nd phalanges, to then insert onto the top, front surface of the coffin bone. It’s aided by the superficial and deep digital flexors which have check ligaments that allow them to behave in a ligamentary fashion when stretched. The network of ligaments encapsulating the foot bones and sesamoids work with the Suspensory ligament as well, to stabilize the foot joints and resist impact and torque. 

However, it’s a common error to credit this system with a bouncy, elastic, and springy gait. In reality, this desired quality is achieved by the relaxed "sproing" of the shoulder sling, not by the Suspensory System (or long, sloping pasterns).

Many sculptures seem to be missing a Suspensory System altogether since its obvious ligamentary bands are absent on the lower leg, particularly the pastern; the Suspensory System is very easily seen on the living horse with healthy legs.

Shoulder Sling

Not only did the horse lose his collarbones during evolution but, at the same time, he also lost a joint directly connecting his scapula to his axial skeleton. That's to say there's no bony connection between the torso and the scapula (and by extension, the foreleg). Instead, the foreleg of the horse is attached to the torso only by a network of bridging muscles, tendons, and ligaments referred to as the “Shoulder Sling." For this reason, equine shoulder motion is fluid, agile, springy, and helps to absorb impact. The muscles forming the Shoulder Sling, for the most part, are the Serratus ventralis, Rhomboideus, Trapezius, and the Pectorals. 

The Serratus ventralis muscle is a continuous fan of muscle tissue whose portions work in conjunction despite often being portrayed in anatomy books as having separate thoracic and cervical portions. It connects the neck and torso directly to the scapula and each "spoke" can fire separately, making this muscle highly finessed. The Serrati can sometimes be seen on a fit animal and do much to convey athleticism in a sculpture. As for an elastic, smooth ride, a relaxed and supple Serrati complex is essential. Indeed, if tensed or braced, the ride will be quite uncomfortable for both horse and rider, even if the pasterns are long and sloping.

The Rhomboideus also consists of a cervical and thoracic portion, yet both act as one system. Simplistically, it arises from the crest and torso to insert on the inner surface of the scapular cartilage and helps to pull the scapula forward and upward, or up and backwards. Also, the cervical portion holds the head up like a boom and can become overdeveloped due to poor riding or schooling that uses headset devices, encourages bracing of the Shoulder Sling, or encourages motion on the forehand. 

The Trapezius, like the Serratus ventralis and Rhomboideus, is often separated into a cervical and thoracic portion, but is still the same system regardless. It’s a thin sheet of muscle, with some aponeurotic qualities, that comes down from the Supraspinous ligament on the crest and wither to attach to the scapular spine and the fascia of the shoulder and foreleg. It draws the shoulder for and aft while also lifting the scapula or, conversely, scrunching down the neck in a “shrugging” gesture. It should be proportionally thin and smooth. While definition is acceptable (the meatier thoracic portion is more noticeable than the cervical portion) any actual bulk in this muscle is undesirable, especially around the withers, sometimes referred to as a “2^nd Level Dressage Bulge." This  indicates it has become overdeveloped with improper riding that encourages motion on the forehand and bracing of the Shoulder Sling. 

The Pectorals essentially connect the scapula to the sternum and ribcage. There are four branches of the Pectoral complex:

• The Posterior Deep pectoral, which forms that familiar “V” in the girth area; it’s a meaty muscle, about 2-3 inches thick. It connects the sternum and ribcage directly to the humerus.
• The Anterior Deep pectoral, also called the Scapula pectoral or the Subclavican Pectoral. It attaches the sternum to the scapula itself via its fascia. It’s an odd pectoral because it’s been co-oped into a locomotor muscle, specifically for protraction of the forelimb, and makes the scapula appear broader than it actually is. It’s often seen on thin-skinned or muscled breeds such as the Arab, TB, and Akhal-Teke, or QH.
• The Anterior Superficial pectoral, those familiar bulging breast muscles on the chest, attaches the sternum to the humerus. 
• The Posterior Superficial pectoral, otherwise known as the Transverse pectoral, is located in the wrinkle area between the forearm and the sternum. It also connects the sternum to the humerus in conjunction with the Anterior Superficial pectoral, but it attaches to the fascia on the inside of the upper forearm, too.

More still, the thoracic portion of the Serrati complex attaches to the scapula with an extremely hard and dense tendon, which feels almost like hard fibrocartilage and acts much like a hip socket (only more flexible). The cervical portion of the Serrati complex also has a slightly smaller version that further helps to support the forehand. 

Another important structure of the Shoulder Sling is the Dorso-scapular ligament that originates at the withers from fascia and attaches to the 3^rd, 4^th and 5^th  thoracic spines. It provides an insertion for the Rhomboideus, Complexus, and Splenius muscles. What's more, a superficial portion is the origin of the Serrates dorsalis (thoracic portion), the deep layer under the Serratus ventralis, which runs along the back. 

The benefit of this design is that it gives a large speed animal tremendous ability for maximum protraction and retraction, abduction, adduction, and rotation without endangering a bony joint. The fluid action of the shoulder also facilitates shock absorption (such as with jumping) while allowing him to shift his balance and direction quickly. This agility at enduring speed is a unique hallmark of Equus, something that's both perfectly functional and perfectly beautiful. 

Another common misconception relating to an elastic gait is that upright pasterns and shoulders result in a choppy stride. But many horses with such conformation are wonderful rides whereas many horses with sloping pasterns and shoulders produce jarring rides. This is because the elastic smooth quality of a stride has little to do with conformational design, but mostly to do with proper schooling that allows the Shoulder Sling muscles to relax and remain supple, producing that desirable springy, bouncy cushion. If the Shoulder Sling muscles (especially the Serrati complex) are made to become tense or contracted due to poor riding, the ride will become choppy and stiff no matter what the horse’s conformation.

A common error in sculpture is pinched shoulder action, or shoulder action that's too hindered or stiff for the action portrayed. The equine shoulder is a dynamic area so we need to give it its due attention in clay. Another typical oversight is not synching the shoulders with the forelegs at all, as though the shoulders were static and it was the forelegs that moved by themselves, or the shoulders are moving in contradiction to the foreleg position. This isn't possible since we know that it's the shoulder (and ultimately, the spine) that moves the foreleg; they cannot move independently.

Reciprocal Apparatus 

The Reciprocal Appratus is unique to equines and its mechanics are of particular interest to artists. It also plays a part in the Stay Apparatus (discussed below).

In order to economize forward motion and utilize its full potential, especially in the chaos of flight, the less the brain has to concentrate on coordinating every detail of motion, the better. Plus, if the upper limbs can automatically produce most of the motion, the more efficient the stride and less distracted the brain is to make an escape. 

If the hindquarter is the horse’s motor, his driving force, then there exists a little autopilot at the wheel, and that's the Reciprocal Apparatus. Taken as a whole, it both economizes and stores energy in a “rubberband-like” system, reducing the need for muscular effort alone to propel the horse forwards. It might have evolved as a function of the Stay Apparatus, but it also produces a handy mechanism for, quite literally, automatic hind leg articulation.

The Reciprocal Apparatus is a complex system that links the loins to the hind toe or, in other words, links the Passive Rebound System to the hindlimb’s Stay Apparatus. The term “Reciproal Apparatus” literally means just that: that every action done in one location has an automatic and simultaneous reaction somewhere else. This reciprocity is based on tendons, ligaments, and yellow ligaments in an automatic, tensionally balanced system that involves the entire hindleg from loins to toe. Working together, the Reciprocal Apparatus literally forces the hock joint to flex in tandem with the stifle (which is automatically flexed in tandem with the femoral joint), making the hindleg articulate just like a drafting lamp, a normal function of hindleg motion and coordination. 

The principle orchestrators of this system are those paralleling components linking the femur to the hock, hind cannon, and foot bones. Viewed from the side, these systems both cross the stifles joint space but run on opposite sides of the tibia, paralleling each other, connecting to opposite sides of the hock. Their reciprocal actions simultaneously compel the hock to flex or extend in synchroniscity with the stifle and femur. Specifically, the tendinous Superficial digital flexor muscle runs from the back end of the femur all the way down to the bottom of the foot, attaching at the hock and coffin bone. It’s assisted by the Achilles tendon which is a twisted cord incorporating the tendons of the hamstrings and Biceps femoris (outermost layer), the Gastrocnemius tendon (middle layer) and the Soleus tendon (deep layer). The Deep digital flexor muscle is also a component, running from the back of the tibia all the way down to the back top aspect of the 2^nd phalange. 

Simplified idea of the Reciprocal Apparatus. It's more complicated than this, but you get the idea.

All of these structures are opposed by the powerful Peroneus tertius muscle that developed into a thick, rubbery yellow ligament. During evolution, as the horse lost his toes and began to depend on the middle digit, this muscle lost its muscle belly to become a yellow ligament (in people, this muscle moves the “pinky toe”). It runs from the front bottom part of the femur to the front of the hindcannon. It’s also opposed by the tendon of the Semitendinosus, which runs from the back of the sacrum and first two tail vertebrae, over the ichium and to the top back portion of the tibia. This involves the Sacro-sciatic ligament which then includes the gluteal fascia above it, which then blends into the dorsal ligament and thereby incorporating the Passive Rebound System. The Tensor fascia latae (TFL) also plays a part, which we’ll explore with the Stay Apparatus.  

But what are the specific effects of this mechanism? Quite simply, whatever the stifle does, so must the hock. If the stifle flexes, the hock must flex and, conversely, if the stifle extends, the hock must extend. In other words, the hind cannon tends to mirror the angle of the femur, more or less. And what governs the stifle? The hamstrings, the Semitendinosus and Semimembranosus (the "semis"), and in due course, the Longissimus dorsi, which then means the loins and LS-joint. So, ultimately, whatever the loins do (which are moved by the spine), the stifle really wants to do as well. This means that if the loins coil, the stifle really wants to flex but if the loins flatten, the stifle really wants to extend. Now it doesn’t have to because the connection isn’t fixed, but this coordinated tendency is the normal functioning of the hindleg. Ultimately, this means that the loins or LS-joint, the governor of the hindquarter, influences stifle motion which then, in turn, dictates hock flexion.

This is clearly seen when a horse either moves in bascule or with a hollow back. The former has great coordination and deep flexion of the joints in the hindleg whereas the latter has a rather uncoordinated “flailing” sequence of protraction and retraction with very little flexion. Another effect to keep in mind is that muscle tension adjusts the tension of the Reciprocal Apparatus, therefore the muscles involved shouldn’t be tense or tight, otherwise quality motion will be compromised or cause a potential injury.

The system is wholly mechanical (about the 6:46 point) and can only change with a severe injury such as ruptured muscles, tendons, or ligaments. For example, flexion of the stifle without flexion of the hock means the Peroneus tertius muscle has ruptured. Or the extension of the stifle and flexion of the hock means the Superficial digital flexor is torn. This is significant for artwork because many pieces fail in this respect, even if by just a margin. The artist should pay close attention to the coordination of their sculpture’s hindlimb in respect to the Reciprocal Apparatus. Remember the hindleg bends like a drafting lamp.

The Reciprocal Apparatus is also associated with string halt, a knot in his Semitendinosus, which inhibits the smooth and proper function of the Reciprocal Apparatus. This causes the horse to flip his hindfoot up to the belly or haul his hindleg abnormally high in protraction while walking or trotting. The interesting thing is that he must coil his loins to canter, which inspires the stifle to flex as well, and therefore circumventing this entire effect. This is why String Halt is rarely seen at the canter.

Errors in the Reciprocal Apparatus are common in sculpture since it's not an often-discussed system when it comes to art work. As a result, we typically see portions of the Apparatus operating independently of each other, which we know cannot happen without severe injury. For example, an extended stifle is paired with a flexed hock, or on the other hand, a standing stifle has an extended hock. In reality, the stifle and hock joints want to mirror each other, something that's consistent despite posture or motion.

Stay Apparatus 

The horse is an intriguing animal, indeed! On the outside, so simple, yet on the inside, we’re finding he’s very complex and and biologically clever. Another of these original features, unique in the animal kingdom, is his Stay Apparatus. And because it, too, has a direct effect on leg mechanics, it’s also of special interest to us.

The horse is a prey animal whose primary defense is immediate flight. That means rest periods can make the animal vulnerable to attack. So evolution outfitted our friend with a unique mechanism that lets him doze off while standing with minimal exertion so he doesn't have to waste precious time standing up in order to take off, since it can be instantly disengaged to allow him to take flight promptly. This is the equine Stay Apparatus. And while it involves his forelimb, it’s particularly well-developed in his hindlimb.

But what does it do? Well, in essence, it locks the joints of the hindleg in extension, preventing them from flexing to turn the entire hindlimb into a passive weight-bearing column. The lower leg is fixed mostly by the suspensory system (dominated by the suspensory ligament) but is helped by many sesamoidean and fetlock ligaments as well. The upper hindlimb is fixed with various tendons, ligaments, ligamentized muscles and muscles that synchronize to create a passive Stay Apparatus that doesn’t need active muscle contraction to function. 

It's extremely important to understand that the Stay Apparatus dictates limb motion in the equine. No aspect of either limb can function independently within itself due to the tight and complex network of ligaments and tendons that establish it, many factors which overlap from the Reciprocal Apparatus. In short, forelimb motion begins with the shoulder (and technically the spine) and not in the foreleg and, likewise, hindlimb motion begins with the femur (again, technically with the spine) and not in the lower hindlimb. That's to say the foreleg cannot act independently of the shoulder, and the hindleg cannot act independently of the femur.

Conditions contrary to the Stay Apparatus are indications of severe injury or chronic debilitating syndromes such as ESAD and DSLD (also known as Equine Systemic Proteoglycan Accumulation, or "ESPA," for short). ESAD (Equine Suspensory Apparatus Dysfunction) is a general term for any failure of the equine suspensory apparatus, making the horse unable to properly support himself in the leg. The condition seems to be caused by several things ranging from severe injury, overstress, poor conformation to genetics. DSLD (Degenerative Suspensory Ligament Desmitis) is a painful degenerative condition of the suspensory ligaments of the legs, inhibiting their support of the horse. It's a bilateral lameness. As the syndrome progresses, most often affecting the hindlimbs, the fetlocks sink increasingly parallel to the ground while straightening the stifles and hocks. Its causes are suspected to be genetically based, but also brought on by overstress or poor conformation.

So let's learn more detail about this...

Stay Apparatus of the Forelimb

The forelimb is stabilized by a symbiotic function of various groups of muscles, tendons, and ligaments. It can’t be mechanically locked like the hindlimb, but it can be made rigid enough to support the forehand with less effort. 

The non-tiring tendinous tissues of the Serratus ventralis muscles, the large tendon of the Biceps brachii that passes over the shoulder joint, the Lacertus fibrosis tendon (from the Biceps brachii to the Extensor carpi radialis) and the Extensor carpi radialis tendon provide stabilization of the shoulder joint against the weight of the body and also function to lock the elbow and, consequently, the knee. 

Its own collateral ligaments, the long medial head of the triceps and the carpal and digital flexors from the humerus also help to stabilize the elbow. The knee is steadied by many structures working together. The Extensor carpi radialis prevents flexion while the Flexor carpi ulnaris and Ulnaris lateralis also help to lock the knee. What's more, the check ligaments of the Superficial and Deep digital flexors apply force to the knee to keep it from collapsing, too. 

 Shoulder mechanisms of the fore limb Stay Apparatus

Fore limb mechanisms of the Stay Apparatus

Therefore, once the elbow is locked, the entire forelimb is also locked until disengaged by shoulder movement to flex the elbow. Though a subtle gesture, one can watch a horse disengage this mechanism by his shifting of weight to one foreleg and articulating his shoulder into flexion.

Predictably then, the reciprocal action of this mechanism makes each portion of the foreleg dependent on the other, just like the hindleg. So whatever the shoulder does, the radius wants to do and whatever the arm does, the metacarpal wants to do; the foreleg joints cannot operate independently of shoulder movement. Again, the foreleg articulates like a drafting lamp.

However, since the forelimb is built like a straight column, weight is sunk directly down into the fetlock, making it somewhat less complicated to stabilize than the hindlimb, which has a “kink” at both the stifle and hock. This may be one of the reasons why the hindleg has a mechanical locking device, which we’ll discuss shortly.

As for stabilization of the fetlock, for both limbs it works by inhibiting over-extension or flexion, produced by three structures functioning together. The first is the suspensory mechanism that includes the Suspensory ligament, proximal sesamoid bones, and the sesamoidean ligaments. The second are the Superficial digital flexor and the Deep digital flexor tendons with their, the third aspect, the Superior check ligament and the inferior check ligament, respectively. Four pastern ligaments and the sesamoidean ligaments also serve to stabilize the pastern. In the foot, the Common digital extensor tendon and the Deep digital flexor tendon stabilize the coffin joint.

Stay Apparatus of the Hindlimb

The Stay Apparatus is particularly developed in the hindlimb, permitting total fixation with little muscular effort, turning the entire limb into a weight-bearing post. The opposing forces of various parts of the hindlimb’s structure (as described for the Reciprocal Apparatus) and the locking ability of the patella are the fundamentals of this system.

The horse’s stifle is equivalent to the human knee. The front bottom part of his femur has a unique feature, a “thumb” or protrusion on the medial epicondyle (the inside ridge).  The femur has a lateral epicondyle as well, forming a groove between it and the medial epicondyle. The patella is an intertendinal bursa (which has ossified) of the powerful Quadriceps femoris muscle and it slides up and down in this groove during motion. However, it’s also lashed to the top of the tibia by three patellar ligaments, those being the Medial, Middle, and Lateral ligaments. They attach to the patella broadly, but to the tibia more clustered together, forming two “V”s between the Medial and Middle ligaments and the Middle and Lateral ligaments. The medial “V” is wider and through it protrudes the “thumb” of the medial epicondyle. Now the Medial ligament connects to the Patella with a sturdy, thick, fibrocartilage “hook” and it’s this gizmo that locks on the “thumb." Laterally, the Tensor fascia latae (TFL), a flat, superficial strap of tendinous muscle, superficially originates at the point of the hip and inserts onto the patella itself laterally and with the patellar ligaments. This is the gist of the Stay Apparatus. 

Mechanisms of the hind leg Stay Apparatus

Hind leg ligaments of the hind leg stifle locking system of the Stay Apparatus

When the horse is standing normally, the patella sits squarely on the front end of femur in its groove with the Medial ligament slipped over the “thumb," allowing it to slide up and down during movement. But, when the horse decides to rest, he contracts his Quadriceps femoris to extend the leg which lifts the Patella to the high point of its groove, slipping the “hook” of the Medial ligament fully behind the “thumb." He then relaxes all of the muscles in that area and the patella is released to slide back down the groove. However, because the Medial ligament is still stuck behind the “thumb," the Patella is stopped and effectively locks the entire hindlimb in extension by stopping all the mechanisms that depend on stifle flexion, bringing the Reciprocal Apparatus into play. Therefore, once the stifle is locked by the patella, the hock is locked as well which locks the entire hindlimb down to the toe. This means he can stand on that leg with no muscle effort whatsoever, easily seen when he rests on his locked hindleg, tilting his pelvis down to flex and relax the other hind leg. 

Now, when he wants to disengage this, the Tensor fascia latae comes into play. The Quadriceps femoris muscle contracts, lifting the Patella. At this moment, the TFL laterally pulls and pops it, with the Medial ligament, off the “thumb," letting it slide back down to unlock the hindleg. This requires perfect timing because the leg must be unlocked before the stifle is flexed. Again, one can spot a horse disengage this mechanism by the shifting of his weight onto the other (often cocked) hindleg and a “popping out” of the locked stifle right before its flexed.

The hindlimb is further stabilized by the superficial digital flexor tendon and the deep digital flexor tendon, both of which run down the back of the hindcannon and into their insertions on the phalangeal joints. The ligaments of the phalangeal joints also lock the hindleg such as the suspensory (or interosseus) ligament and the sesamoidean ligaments.

Put it all together and this allows a horse to sleep while standing up with little or no activity of the major muscles. That is, instead of muscle bellies, work is bypassed to non-tiring connective tissue, the tendons and ligaments, and the horse can stand like this indefinitely. However, this system also allows immediate action if the animal is threatened by danger. It takes but a moment and an easy action to unlock the system, and the muscles are ready to work immediately so the animal can get away quickly. 

But here’s the sticky part, literally. The patella slides up and down the entire length of its groove with every stride, sliding up for extension and down for flexion. Specifically, it reaches the high point of its groove during each moment of maximum protraction and retraction, two moments of every stride. This means that the patella comes precariously close to involuntarily locking twice during every stride. The only means to avoid this, at every stride, is for the TFL to govern the position of the Patella by contracting to laterally pop it away from the “thumb." In a normal horse, all these complicated motions are perfectly coordinated together into an instantaneous, smooth, and beautiful system that requires a minimum of effort.

And, again, because of this synchronized network, no aspect of the hindlimb can function independently within itself. Essentially, hindlimb motion is a product of femoral motion; the hindlimb cannot move independently of the femur or, in other words, the hock cannot move independently of the stifle. And ultimately, femoral motion is governed by the loins, which is governed by the spine. All this means that whatever the femur does, the metatarsal really wants to do, too.

However, he can run into trouble, most notably, when people impose demands on him that aren’t sympathetic to this system. If his timing is made to be off by poor horsemanship, he can accidentally lock his stifle, gravely injuring himself by tearing structures of the stifle joint, literally blowing out his knee. But if this doesn’t happen and his tissues don’t rupture, the patella ligaments can become stretched to cause chronic locking. 

But what makes his stifles lock involuntarily? Largely because of a non-active TFL, one that’s kept from doing its job. But what de-activates the TFL? We have already seen how the Reciprocal Apparatus works and how it links the articular relationships between the loin and hind toe. So by making the horse move within a series of “opposite” relationships, locking the Patella becomes practically inevitable. For example, making him move under saddle with a hollow back and long hind leg strides, or tightly coiled with long protraction strides are the usual culprits.

Indeed, riding a horse in a hollowback posture continually makes him prone to locking his stifle. A hollowback means he has flattened his loins, tensing his back in protection, which inspires the stifles to extend longer and deeper per stride as the hindleg is pushed further back in retraction, which also disrupts his natural coordination. In contrast, however, riding in bascule inspires the stifles to flex, avoiding this problem altogether. Why? Because this form of riding uses his natural systems and normal coordination to carry the rider. This is a clue why stifle locking is rare at the canter or gallop…because the horse must automatically coil his loins to achieve these gaits.

Permitting him to move crooked also contributes to stifle locking. “Leaners” habitually carry their spine in a curve, towards the side they favor. This means that the hindleg outside the curve has an unnaturally longer retraction length to travel, making that patella slide to the highpoint with every stride. And since his body is askew and uncoordinated, this can lead to a disruption it the functions of the TFL, causing involuntary locking of that patella. Post-legged conformation can initiate fixation involuntarily as well, having a more “extended” posture to the leg. The Stay Apparatus can also be artificially fixed in place by hand with enough skill.

Equine sculpture suffers a lot of ESAD and DSLD, often accompanied by coon feet, because many artists don't understand the authority of the Stay Apparatus in forelimb and hindlimb motion, or misunderstand the proper angles of the foot in relation to the limb. Many artists simply don't understand what constitutes a bona fide "straight" hind leg, or the mechanisms behind the Stay Apparatus and Reciprocal Apparatus.

Standing shoulders with articulated forelegs and standing femurs with articulated hind legs are common as a result. Articulated shoulders with standing forelegs or articulating femurs with standing hind legs also occur, but more rarely. On the other hand, sometimes the shoulders or femurs aren't articulated enough to explain the leg motion or visa versa. Likewise, sometimes the hock or knee, and corresponding fetlocks, aren't articulated enough for the joint angles high up on the limbs, producing a dangling effect rather than an articulated one.

Conclusion to Part IV

Phew! That was a lot to digest, wasn't it? And this is just the tip of the ice berg! There's so much more to learn about the horse's biological systems, and it's a blast to discover them. It's so easy to take these complex systems for granted when we look at a horse, since we can be so distracted by his beauty and personality. But when we begin to perceive his biology at work, he becomes that much more of a living marvel. We also gain insight in how to better design our sculptures to mimic life more authentically, giving our pieces more fluidity and nimble grace, qualities characteristic of equine motion.

So until next time...all systems go!

"Once an artist gets it in his mind that it's a blooming adventure, then, and only then, everything falls into place and starts to work."

~ Joe Joseph P. Blodgett

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part IV, Systems