Monday, August 29, 2016

Equine Anatomy and Biomechanics: A Primer of Equine Engineering Part III, Tissues

Fascia forms a continuous network throughout the horse's body.


Hello again! Welcome back to this 17-part series exploring equine anatomy and biomechanics on a more in-depth level and from an artistic point of view. In Part I, we had an overview of the series, then in Part II, we explored some basic terminology. Now in this Part III, we'll have a basic discussion about tissues. Keep in mind that these explorations are by no means complete, but it's hoped that they'll inspire proactive education. Our subject is a marvel of bioengineering, and learning how and why is an exciting proposition!

Now one may wonder why we're discussing tissues, something that seems a bit too nit-picky for our overall subject. But the truth is we need to know some basics in order to understand the structures themselves which, in turn, helps us understand his construction better and that has consequences for what we choose to portray in our clay. 

So let's move onto tissues...let's go!... 


Simplified, tissue is a grouping of like cells, and tissues grouped together make an organ, and organs grouped together make a body. For simplicity’s sake, the tissues have been categorized into nine groups: bone, epithelial, connective, muscle, ligaments, skin, fat, blood, and horn or hair. However, the information presented here isn’t complete, but intended merely to provide a working overview of the structures involved.

Types of Tissues


Bone is a hard and complex structure that forms the skeleton of the horse. However, it's not static, but constantly changing in density and shape according to the pressures, influences, and stresses on them. And while not instigators of motion, they are the passive supports for muscles and flesh; they're what flesh activates into motion. Plus, by their structure, they help to create fixed parameters of motion distinctive to the genus. 

During fetal development the body first builds a 3D web-like structure (or precursor bone), yet mystery remains as to how the DNA knows where to place each precursor bone cell to correctly built a fetus. It’s the collagen cells, the osteoblasts, which create the precursor bone by secreting collagen fibers (and when they do, they’re called fibroblasts). The fibroblasts then stop making collagen fibers and start creating cartilage (and are now called cartioblasts) to overlay onto the precursor bone. Yet the fiber matrix remains, and is lived in and used by the Osteoblasts. Lastly, the Osteoblasts dissolve and re-absorb the cartilage and in its place, lay down bone via calci-apatite, the crystalline structure that makes bones hard. It’s the collagen matrix that holds the calci-apatite crystals in place, and the matrix will always remain as long as it’s maintained by the Osteoblasts. As a matter of fact, the Haversian Canals of the bones follow the fibers of the old cartilage matrix. Being so, each bone found in a mammal starts out as cartilage save for a few bones in the head that grow directly from a membrane. 

The shaft of a bone is referred to as a diaphysis and either end is called an epiphysis, which is sheathed in articular or hyaline cartilage. This articular cartilage usually accentuates the curvatures of the joint and are non-vasular, smooth, and have a blue tinge. They work to greatly reduce friction and help to absorb impact. In addition, each bone is encased in periosteum, a fibrous, strong connective tissue rich in circulation and full of Osteoblasts, the living cells that nourish, create, and heal bone. In reality, however, the periosteum can really be considered bone fascia. Strong little tissues called Sharpey’s Fibers are the actual “velcro” that glues flesh to the periosteum and the periosteum to the bone, even embedding into the bone. Stress increases the density of these Sharpey’s fibers, resulting in greater strength in that area. 

Via the inter-muscular septa, a kind of fascia grows from the periosteum as a series of networked sheets that encase, shape, stabilize, and divide the major muscle bodies. This makes periosteum and fascia a continuous presence throughout the entire body. It also means that muscles don’t actually pull on the bones but on the periosteum; it’s the actual tissue that enables motion. Also since ligaments and tendons attach to it as well, the periosteum effectively helps to provide the means to glue the entire skeleton together. Unfortunately, however, its exactly this that's stripped away (along with the hide) to reveal the underlying musculature, giving us a false sense of structure and mechanics.

Oxygen and sugar substances go into the bone via the arteries to the periosteum and reach the outermost Osteoblasts through diffusion. The Osteoblasts in the bone live in the lacunae and extend their little arms through the canaliculi to each other, linking each Osteoblast to the others. These outermost Osteoblasts use what they need, then send the extra via their little arms to the next Osteoblasts who do the same until they all get the nourishment they need (aren't they cute!? Linkin' their little arms across the matrix! It's all about the little guys!). In this manner, bone lives and is nourished as the Osteoblasts receive nourishment to maintain the collagen and mineral substances in the bone. But when the animal dies, his Osteoblasts die, too, leaving just the unstable calci-apatite. This is the reason why bones disintegrate over time and need varnishing for preservation. 

Inside the bone is a minute framework of architectural buttresses called trabeculae that align themselves to gravity and the pressures of everyday stresses, making the bone very strong internally. For example, in America racehorses run counterclockwise and so their trabeculae align along the left side of their leg bones, shoring up the side with the most pressure. In contrast, overseas where the horses run mostly clockwise, the trabeculae are aligned on the right side. Now in an environment lacking gravity, this creates a random order to the trabeculae and such a bone will break very easily when back on Earth. Truly, the trabeculae have a lot to do with the durability and resilience of bones.

In the immature horse, bones also contain growth plates, zones of cartilaginous “bone” not yet ossified. They’re located behind the growth plates on either end, between the diaphysis and the epiphysis. These are the only places where the bone can grow longer as the horse matures. The bone must also grow in diameter to maintain a suitable proportion to the increasing length and this is achieved by the periosteum. Therefore, length is added by the growth plates and width by the periosteum in a lamellar fashion, like growth rings on a tree. 

Bones come in various shapes and sizes, depending on their location and function. Wherever a bone meets another bone, a joint is formed, and its shape is dependent on its function. The equine skeleton is totally unique to equines, making it important for an equine artist to understand. And being unique means unique musculature and topography, and also unique movement.


These tissues line systems such as the mouth, cornea, ducts, tubes, respiratory tract, reproductive tract, etc. and their main purpose is to maintain moisture by secreting fluids, mostly mucus. Some of these tissues are ciliated (having little hairs) that move particles out of the passage to keep it clear, or move secretions around (by waving in sequence). They also serve as barriers between systems, or between the body and the outside environment.

Epithelial tissues have tremendous healing powers and can withstand quite a bit of wear. This is necessary for survival. Indeed, it’s important to maintain eating habits, digestion, sight, etc.

A common term “hard mouth” implies that calluses have developed in the horse’s mouth. However, since his mouth is lined with epithelial tissue, this isn’t possible; calluses simply don’t happen in the mouth. What's actually happening when the bit is hurting him is to protect himself with a stiff neck or taking the bit in his teeth. Only actual nerve damage will create numbness in the mouth and subsequent indifference to the bit.

Connective Tissues

These tissues share the same embryological origin and are made of different forms of collagen, so they’re all considered connective tissues. They play a significant role in the function of the horse’s body and actually comprise the greatest percentage of body tissues. They come is myriad forms, involve the entire body by encasing or connecting everything, and run the gamut of characteristics. 

All connective tissue will attempt to ossify (to become bone) under the right circumstances of constant stress, which stimulates the collagen cells to secrete calcium apatite, calcifying the area into ringbone, for example.

Cartilage is a connective tissue, and is a composite that’s dense, tough, rubbery, and elastic. It's used in joints to buffer friction or form flexible body features such as the nostrils, ears, scapular cartilage, etc. It can ossify with injury, maturity, or with chronic conditions such as arthritis. Fibro-cartilage is quite common in the body, even the short pastern bone has a complement for the tendinous attachments of the flexor muscles. Menisci are plates of fibro-cartilage often inside joints, and intra-articular cartilage often mediates between mismatched articular surfaces such as in the stifle. Intervertebral fibro-cartilages are discs that lie between the vertebrae.

Bursar ("bursa" for singular) are composite structures made up of tough collagen fibers encasing a wad of collagen gel formed into sheaths, sacs, or pillows. When a tendon transverses a bone or other hard surfaces, bursae underlay them or are even embedded in them, becoming an inter-tendinal structure. They act as viscous pillows to dampen friction, but like all connective tissue, tend to ossify with wear and tear. The shoulder joint has the largest bursa in the horse’s body, which can calcify with age or with the stress of a jumping career. In fact, some important bones in the horse’s body actually started, evolutionarily speaking, as bursar. A true bone has periosteum, yet the sesamoids “bones”the foot sesamoids, patella, and navicularlack this essential component. This is because they really aren’t bones, but actually inter-tendinal bursae that have ossified during evolution. This is also why injuries to these structures are so stubborn to heal since they lack a circulation-rich periosteum. Another type of bursa is a tendon sheath, which are thin walled, and fluid-filled tubes that protect the tendons from friction as they pass over joints, such as the carpus or tarsus.

Fascia (pl. fasciae) comes in many forms and is an extremely important (and typically overlooked) part of the body. Overall, it’s a tough, connective, supportive band, layer, envelope, or sheet encasing all aspects of the body from muscles to bones to organs, essentially connecting everything to its neighbor and in this way, forms a whole system within the body. Indeed, it's what gives muscles their shape and what "glues" the body together. 

There are five basic different types of fascia: “fiber” fascia, “bubble” fascia, “sheet” fascia, “jello” fascia, and "wrapping" fascia. Fiber fascia is like a dense web of fibers or strands. It’s often used to lash things together like skin to the body. Bubble fascia is similar, but has multitudes of thin membranes formed into little pockets full of oil or other viscous fluids. This kind of fascia is prevalent in areas characterized by friction and to function as a heat sink, such as under the scapula and around the muscles of the hindquarter. Fat is often deposited in this type of fascia, which can lead to cellulite. Sheet fascia (also called an aponeurosis) is a non-stretchy sheet of matted collagen fibers often used to help support structures such as the gut and legs. Jello fascia is a loose mushy tissue often found wrapping nerves and organs. Wrapping fascia, the most common, encases everything from organs to bones to muscles, knitting the entire body together and helping to give structures their shape, being closely connected to the deep fascia (see below).

The superficial or subcutaneous fascia, typically fiber fascia, glues the skin to the underlying features and contains the voluntary cutaneous (skin) muscles known as the “fly shakers." The development of the fly-shaker muscles vary with individual and breed, but tend to be more developed over the body and less so on the limbs. The robustness of the fly-shaker muscles is also more a factor of genetics than being built up by actual use. These cutaneous muscles are often referred to as cutaneous faciei, cutaneous labiorum, cutaneous colli, cutaneous omo-brachialis, and the cutaneous trunk, depending where it's located on the body. The cutaneous fasciei lies on the face over the Masseter, but a branch projects forwards to the Orbicularis oris where it becomes the cutaneous labiorum which helps to pull back the angle of the mouth (and is therefore sometimes called the Retractor anguli oris). The cutaneous colli (also called the Cervical panniculus) is located on the neck, originating on the sternum where it'
s thick to thin as it radiates outwards, often blending with that of the head and lateral sides of the neck. It forms that sometimes seen “V” at the base of the neck, above the pectorals, as seen from the front. The cutaneous omo-brachialis drapes the arm and shoulder and continues to the cutaneous trunci (also called the Panniculus carnosus, sometimes in junction with the cutaneous colli), the abdominal portion. This part sheaths much of the torso from the middle of the back ventrally to the flank, where it forms a fold at the stifle and blends into the stifle’s fascia. It has a thin tendon which flows with the Posterior deep pectoral muscle to the medial tuberosity of the humerus and also blending with the tendon of the Latissimus dorsi. It also forms the fold of skin at the elbow with the cutaneous omo-brachialis. Ventrally, the two sheets meet about four inches apart and project forwards, to blend with the Posterior deep pectoral, and contains the external thoracic vein (or “Spur vein,” in the girth area).

The deep fascia, typically aponeurotic fascia, has more functions and is consequently very complex, more dense, and highly developed on the limbs. It sheaths muscles in a close-fitting septa (sleeve), separating and directing muscle function along specific parameters while also attaching to the periosteum and thereby providing routes for vessels, nerves, and the lymphatic system. It also encases joints, blends with ligaments and tendon sheaths, and is tough enough to also serve as direct attachment for some important muscles. It  facilitates definition and gives shape to muscle masses, too, while also acting as a powerful supportive tissue for viscera (guts). Being well developed in the legs, it provides definition, organization, and stabilization of limb features and mechanisms. 

In essence, fascia is the connective, supportive, organizational, and bracing structure of the entire body, providing a continuous, complex network of binding tissue that maintains order and links every aspect of the body to everything else. How strange that it's treated with such a cavalier attitude in dissection! And stranger still is how it's ignored in sculpture when it provides so much textural detail to the hide and flesh.


Muscles are usually attached directly to bone (well, to the bone's periosteum), but they can also attach to cartilage, fascia, ligaments, and even skin. All muscles have an origin and an insertion. The origin refers to where a muscle attaches to bone proximally while the insertion pertains to where a muscle attaches to bone distally. The origin is also usually located on the more immobile site and is attached more strongly. Knowing the origin and insertion of a muscle provides better insight into its effects on motion. The origin of a muscle is referred to as the "head" and the body of the muscle is called the "belly." Some muscles have multiple heads, leading to the terms “biceps” and “triceps," both based on Latin roots. In contrast, a "digastric" muscle has one tendon with two bellies. 

Muscle is made up of fibers that animate the skeleton into motion. Muscles can only contract (pull) or relax (go slack), and so they're formed in reciprocal pairs that cross a joint surface to create articulations. When a muscle contracts and its reciprocal relaxes, the bone is pulled in that direction, creating movement. The reverse happens to create the opposite motion. When both are contracted, either no motion occurs, motion is dampened, or other effects such as lifting of the torso or stabilization of an area results. Muscles also help to stabilize the entire body by making constant muscle adjustments to posture and body alignments.

In this way, muscles (with their tendons) produce motion by essentially acting as a pulley system, converting segments of bone into levers. That in mind, it’s rather amazing that the complexity and grace of equine motion is accomplished by just a system of pulleys! 

And in regards to this, the equine has a peculiar muscular and skeletal relationship typical of a prey animal, in respect to the legs. Specifically, the limbs of the horse have developed into a system of long levers to facilitate the flight response. Muscles are placed high on the limbs, above the knee and hock, and their actions are transferred to the lower portions of the limbs via a pulley system of tendons. In other words, there exist no muscles below the knee or hock. This means that relatively little motion at the top of the limb results in much bigger action at the end of the limb. In this way, this system removes excess weight from the lower leg to maximize speed, endurance, and efficiency of stride while also minimizing expended energy to sustain speedy flight from predators. The equine is a perfectly built running machine, something we'll explore further in this series.

As for tendons, they attach muscle to bone, and are strong, elastic, and capable of storing energy when stretched. They may be thick and round or oval, or thin and flat. A key element of the pulley system, they transfer the action of the muscles to the bones and joints. Think of them as extensions or servos of the muscles themselves. They also function as joint stabilizers and shock absorbers. The elastic rebound of tendons contributes much to equine motion, too, by releasing the stored energy in the muscle bellies. The important thing to remember is that tendons aren’t separate components to muscles; the muscle and its tendon aren’t different things. In reality, the tendons are the muscles themselves, portions which have simply lost their muscle bellies and retained their elastin. That's to say the tendon is simply the muscle continued in a different form. That said, tendons are so strong that extreme stress is more likely to rupture the muscle belly than the tendon itself.

Some terms associated with muscles are:
  • Aponeurosis: Large sheet-like tendinous or fascia expansion that serves to connect muscle with the part it moves. The belly has a large aponeurosis.
  • Visceral or smooth: Muscles of the organs (i.e. esophagus, stomach etc.), and are involuntary with no striations.
  • Cardiac: Heart muscle, which is striated, involuntary, and contains intercalated discs.
  • Skeletal: Muscles of the skeletal system (movement and posture), which are voluntary and striated.
  • Epaxial: Muscles found above the transverse processes of the lumbar vertebrae. 
  • Hypaxial: Muscles found below the transverse processes (such as the abdominal muscles).
  • Origin: The proximal attachment of a muscle, which is less movable.
  • Insertion: Usually more distal attachment of a muscle, which tends to be more movable.
  • Intrinsic: A muscle which has both an origin and insertion on one bone (such as the brachial or infraspinatus).
  • Extrinsic: A muscle which attaches the limb to another part of the body (i.e. the brachiocephalicus or pectorals).
  • Antagonistic: The paired muscle that opposes the movement of a prime mover muscle (i.e. the long head of the triceps against the biceps brachii in the flexion of the elbow).
  • Body or belly: The main fleshy portion of a muscle.
  • Tendon: The fibrous strands that derive from muscle bellies.
Muscle is typically fusiform in shape, a belly with tapered tendinous ends. It’s made of up bundles of myofilaments or muscle fibers or striations. These are then grouped together into myofibrils composed of two kinds of inter-digitated protein strands, actin and myosin, that contract the muscle when electro-chemically stimulated by the brain. The thicker red striations are the myosin and the thinner white or yellow striations are the actin, and they’re chemically very attracted to each other. Each complementary section of actin and myosin is bordered by endplates (otherwise known as Z-lines) when, during contraction, pull closer together. A sarcomere is the minimal contractor unit comprised of a section of actin and myosin, endplate to endplate. Contraction occurs with the introduction of calcium ions (in a chemical reaction too complicated for this discussion) so that the endplates are pulled together. This process cannot be stopped once it has started and ends only when the lengths of the actin and myosin have bumped against the endplates, which takes a few milliseconds. The energy for this process is supplied as ATP (adenosine triphosphate)the immediate energy source for muscle. In response, the calcium ions are quickly pumped out thereby inhibiting attraction between actin and myosin, resulting in relaxation. It’s the synchronized contraction and relaxation of a muscle’s millions of sarcomeres that initiates the skeleton into motion.

The only way to mediate the strength of the contraction is for the brain to selectively stimulate different percentages of the sarcomeres. In other words, a soft or gentle motion entails only a few sarcomeres whereas a strong powerful contraction involves a large number of sarcomeres. Likewise, coordination can be thought of as the use of muscle stimulation to produce the smoothest and most efficient motion.

Muscle is sheathed in fascia, which gives it its shape and connects it to its neighbors. There are different muscle types, depending on their function, most basically either voluntary or involuntary. Involuntary muscles, also termed “smooth muscles," are those the horse cannot control such as those in his circulatory system, digestive system, or other structures associated with an automatic response. Voluntary muscles, also called “skeletal muscles," are those the horse can control such as his locomotor muscles and his skin muscles, and are the most abundant muscle in the body. Muscle tissue includes cardiac muscle as well.

Some muscles are “fast twitch” muscles and some are “slow twitch” muscles. These terms refer to the capabilities each have for metabolizing fuel. Fast twitch muscle fiber has a very fast contraction speed designed for immediacy and high intensity, for rapid bursts of motion over a short period of time. There are actually two types of fast twitch muscles: intermediate and white fiber. Both function in an anaerobic environment (without oxygen) for a limited time since each fatigues rather quickly. Anaerobic motion uses sugars and fatty acids as the main fuel sources and usually entails a heartbeat over 150 beats per minute. Flat racing is a good example of the use of fast twitch muscle fibers. Different breeds are thought to possess different ratios of these muscles fibers. Draft horses are thought to have more slow twitch fibers and Quarter horses and Thoroughbreds are believed to have more fast twitch fibers. Conditioning cannot increase the number of these different muscle fibers, but it can enlarge them.

In contrast, slow twitch muscle fibers have a slow contraction rate and use oxygen and fatty acids for fuel, an aerobic environment. Sometimes referred to as red fibers, slow twitch fibers are designed for staying power and endurance with a heartbeat usually under 150 beats per minute. Endurance racing is a good example of the use of slow twitch muscles.

Tonus refers to the firmness of muscles in their resting state. A “spongy boing” of muscles is described as having low tonus whereas a “firm boing” is referred to as high tonus. This “boing” is attributed to the amount of elastin contained in the muscles, which can be an inherited trait. Indeed, some breeds are typified by low tonus such as Friesians (among the lowest), Morgans, Warmbloods, Saddlebreds, Walkers, etc. On the other hand, Drafts have medium tonus while Akhal-Tekes, Thoroughbreds, and Arabians have a high resting tonus. Overall, tonus may be linked to a breed’s reactionary or sensitive nature. 

A handy way to visualize the overall organization of muscles is to classify them as body (axial) muscles, limb (appendicular) muscles, or bridging (linking) muscles. To clarify, body muscles only entail the bones of the body such as the Masseter, poll muscles, Multifidis muscles, etc. Likewise, limb muscles only deal with the bones of the limbs such as the Supraspinatus, Infraspinatus, Gastrocnemius, etc. However, bridging muscles are those that mediate or link the muscular actions of the body muscles and limb muscles thereby creating actual directional motion such as the Longissimus dorsi, Serrati complex, Psoas, Semitendinosus, etc. For this reason, they're also the most prone to injury and the most targeted in body working.


Often found at joints, ligaments are remarkably tough fibrous straps that typically bind bone to bone, holding the joints together (though there some important exceptions which we’ll discover in a bit)While they usually don’t have an active role in muscle function, they function as important skeletal and joint stabilizers, and help govern the mechanics of the joints by limiting movement in specific ways.

Many joints, especially in the legs, have collateral ligaments binding the joint as paired straps. Annual ligaments (or ring ligaments) encircle a joint to lash accompanying tendons down to further stabilize the joint. Cruciate ligaments (derived from the Latin work for “cross”) are those that cross each other’s path to stabilize a joint, as found within the stifle joint, for example. Interspinous ligaments connect each of the spines of the vertebrae, and intertransverse ligaments connect neighboring transverse processes. Periarticular ligaments lie inside the layers of the joint capsules. The ventral longitudinal ligament runs on the bottom surface of the spinal column running from the middle of the back to the underside of the sacrum where it blends with the sacral periosteum (it’s strongest in the lumbar span). The dorsal longitudinal ligament lies underneath the spinal column, on the bottom of the vertebral canal from the sacrum to the Axis bone and is strongly united with the intervertebral fibro-cartilages. Actually, there’re many types of ligaments, typically named by location or function, and all serve critical functions. It’s also important for the artist to know that ligaments can occlude the true shape of the bones and also serve as important surface landmarks, particularly in the legs.

Nonetheless, there are some important ligaments we should specifically know due to their important influence on motion. These ligaments are the Supraspinous, Nuchal, Sacro-iliac, Sacro-sciatic, Suspensory, Check, Stifle, Pubio-femoral, Teres, and a cascade of leg ligaments.

In the torso, the Supraspinous, Nuchal, and Sacro-sciatic ligaments are of particular interest. These three major ligaments help to form what is known as the dorsal ligament system, or the Passive Rebound System of the horse’s body (which we'll discuss in Part IV). Between them, and other structures we’ll discuss later, the bones from the poll to hindfoot are linked, creating an important mechanism for the motion of his spine, pelvis, neck, head, legs, and tailbone. 

The Supraspinous (or Dorsal ligament) is a thick powerful cord that runs along either side of the back, along the top of the spinous processes, from the sacrum to the withers, thusly uniting all the spinous processes of the vertebra. It also provides attachment for several muscles of the torso. Towards the sacrum, it’s made primarily of inelastic white tissue but as it approaches the withers, it becomes increasingly elastic and adopts the characteristics of the funicular portion of the Nuchal ligament which it blends with. At the withers, it flattens to form a broad shield, about 4-5 inches wide, extending from either side almost to the scapular cartilage, and becomes modified to form the Nuchal ligament. Because the Sacrum is fused, there’s no pressing need for the Supraspinous to exist entirely there nor cross over the lumbo-sacral joint (or "LS joint," discussed later). So at this joint, it has a gap, but its influence is carried over to the sacrum and tail via the deep layers of the skin, producing the lovely arched "rainbow tail" of bascule.

The Nuchal ligament (or Cervical ligament), which is actually an extension of the Supraspinous ligament, links the head and neck with the torso. At rest, it supports the head and neck with minimal effort. It actually has two sections, a funicular portion and a lamellar portion. The funicular portion erupts from the withers at T4 and continues up the crest of the neck to attach at the back of the skull at the occipital tuberosity, forming the crest. The lamellar portion entails two bilateral sheets, separated between by loose connective tissue and fascia, reaching down and forwards from the funicular portion to attach with digitations to the spinous processes of C2-C6, except C1 (Atlas) and C7, and to the spinous processes of T2-T3.

However, the Nuchal ligament (like the Suspensory ligament) isn’t really a ligament. Actually, many aspects of the horse’s body don’t fall into neat pigeonholing. True ligaments are white, strong, stiff, and non-stretchy with collagen fibers organized in a parallel matrix. Yet the Nuchal ligament is a “yellow ligament” since it has more collagen fibers loaded with elastin (stretchy spiral fibers), making it quite elastic and rubber-like. Elastin is comprised of perimysium and epimysium (which are also major components of tendons), yet real ligaments in fact have very little, if any, of these components. Also, tendons are flat whereas muscle is rounded. Surprisingly, yellow ligaments are rounded or episoidal in x-section (like cables). So in actuality, these “yellow ligaments” really used to be muscles that have lost their muscle bellies, leaving just the elastic aspects. In other words, the Nuchal ligament used to be a muscle. Then the old adage that  “ligaments connect bone to bone” isn’t quite true since yellow “ligaments,” that were once muscles, also connect bone to bone. 

The Sacro-sciatic ligament fills in the top hollows of the pelvis like a roof, from the sacrum to the sciatic notch of the femur and interior edge of the pelvis. It’s one of the primary braces of the pelvis by lashing each lateral aspect of the sacral crest and transverse process of CG1 and CG2 (tail vertebrae) to the pelvis. The tendon of the Semitendinosus muscle also attaches to this ligament, making it part of the Passive Rebound System than links the hind leg to the poll.

The Sacro-Illiac joint (SI Joint) (or the Lateral Sacro-sciatic ligament) lashes the internal aspect of the ilium to the wings of the sacrum, blending with the Supraspinous ligament. It’s not a closed or fused joint because nature has intended it to float a bit, actually creating a suspensorium. However, the thrust of the hindlimb is continually trying to tear the pelvis off the sacrum and spine since the attachment is from below the wings of the Illium and not from above. So this junction can be vulnerable to stress from age, poor riding, or injury which can cause ossifications or fusion. Sometimes a stretch or tear in these ligamentary bindings can reseat the pelvis crooked to the sacrum, resulting in a horse with a permanently crooked pelvis to the spine, unable to ever move straight again. Also, sometimes, an injury to the Sacro-Illiac ligament can cause atrophy of certain muscles overlaying the area as is often the case with a Jumper’s Bump, Hunter’s Bump, or Racker’s Bump.

The Suspensory ligament (or Interosseous ligament or muscle) is a broad strap of strong tissue that helps to support the back of the limbs. Specifically, it’s a primary source of support for the fetlock and pastern, protecting those joints from overextension. It’s also part of the Stay Apparatus (discussed in Part IV) and helps to support the horse’s weight and absorb shock. The sesamoids, to which it attaches, are thought to brace it and disperse stress as it wraps around the joint. The Suspensory ligament is clearly seen on the pastern. Like the Nuchal ligament, the Suspensory ligament has a similar story. When the horse was evolving and losing his toes, the interosseous muscles in his “paw," those muscles between the digits, were being lost as well. As the horse lost digit II and IV (to stand on digit III), the interosseous muscles between these digits morphed into the Suspensory Ligament. Indeed, this “ligament” really isn’t a ligament, and like the Nuchal is a yellow ligament and more elastic. The Suspensory even still has some muscle fibers. So the Suspensory “ligament” is really an ancient, morphed Interosseous muscle. Actually, think of the horse’s leg and the Suspensory ligament as the result of cursorial evolution on the human hand. Indeed, a fetal horse at one point during development has three toes with Interosseous muscles that eventually turn into one digit with a Suspensory Ligament!

The check ligaments of the legs deserve attention because of their structure and function. Also, they're additional curious exceptions to the rule. Unlike typical ligaments, which connect bone to bone, the check ligaments of both the foreleg and hindleg run from bone to tendon. The forelimb’s Superficial flexor tendon has a check ligament above the knee, coming from the radius, called the Superior Check ligament (or Radial or Proximal ligament). Likewise, the forelimb’s Deep flexor tendon also has a check ligament, the Inferior Check ligament (or Distal or Subcarpal ligament), coming from the carpals. The hindlimb’s Deep flexor tendon has a check ligament below the hock, called the Tarsal check ligament. However, this ligament is much thinner than that of the foreleg and may even be absent. Nevertheless, the hindleg’s Superficial digital flexor compensates for this by its firm attachment to the calcaneous, despite not having a check ligament itself. 

All that said, however, in reality, the nature of the check ligaments is far more organic than this over-simplification. In the living animal, a whole series of check-like tissues run from the bone to the tendons in a cascade of support. In other words, we don't have discreet straps of check ligaments like we see on an anatomy chart, but organic, interwoven filaments running up and down the tendon.

Biomechanically, this arrangement of connecting bone to tendon makes these tendons function as ligaments when extension or flexion reaches a certain point by protecting the muscles above the attachment. Even so, popular belief maintains that these check ligaments hold the tendons to the bone when, in truth, their true function is to pull the bone onto the tendon, particularly in the forelegs. In doing so, they keep the knee from buckling forwards ("over at the knee"), pulling it towards the tendons which want to be straight (the shortest distance between two points is a straight line). This allows the leg to remain in alignment with minimum effort. Indeed, Buck Knees (or Bucked Shins) is rarely an inherited condition (which would be a rare misalignment or malformation of the carpals). Instead, it's an injury to the check ligaments, eliminating their support system which causes the knee to buckle forwards. (It can also be caused or exacerbated by a continual contraction of the flexor tendons, pushing the knee forwards.) The extensors of the legs, in turn, have to compensate for something for which they were never designed and they tire quickly, causing the forelegs to tremble with fatigue. Such an injury is also typified by broken angles of the foot bones and perhaps a dish in his hoof wall, which would be expected with a disruption in the normal alignment of the entire bony column of the foreleg. 

However, the perception of Bucked Knees can be symptomatic of a chronic misinterpretation. For example, American breeders tend to have forgotten how to breed good forelegs whereas the Europeans (especially the Germans) haven’t. Specifically, Europeans produce forelegs in which the radius meets the carpals in a 90˚ plumb, the correct alignment. In contrast, American trends favor all degrees of calf-knees, typically with small joints and light bone to boot. Thusly, the forelegs of European horses seem “over at the knee” to many Americans. Nevertheless, a correctly plumb foreleg, one which looks “over at the knee” to most Americans, lacks the broken alignments of the foot bones since it has the normal and desirable construction. So this relates to the idea that an “over at the knee” horse has sound legs for jumping, a misconception held by many Americans. They don’t understand that these so-called “over at the knee” horses actually have correct alignments. So in a desperate want for a good set of jumping legs, Americans can make the mistake of choosing horses with actual Bucked Knees. In reality, however, a Bucked Knee horse should never be jumped because he’s suffering a serious injury. Heck, he can barely stand! 

This lovely mare demonstrates correct alignment of the cannons to the radius. She's not "over at the knee," but properly aligned; no calf-knees here!

Similarly, calf-knees are ubiquitous in equine sculpture as artists misinterpret correct alignment. Almost every sculpture has them to some degree. They can also be produced by the nature of certain media, such as we see in fire media, especially porcelain, which causes the long limbs of the forelegs to curve forwards at the hoof when brought to a mature fire and the porcelain liquifies momentarily. Unless there's compensation happening, such as pushing the hoof backwards at the knee in greenware, the longitudinal shrinking of the foreleg column will produce a calf-knee. The forelegs of newborns and young foals may appear even more Over At The Knee since their forelegs have been folded in the uterus for so long, and they need to stretch out.

Anyway, moving onwards...the Pubio-femoral ligament or Accessory femoral ligament deserves special notice and is particular to equines. It effectively limits the motion of the femur from direct outward, sideways motion, making it one of the reasons why a horse can’t kick straight sideways like a cow. 
There’s also a round ligament inside the hip socket, the Teres ligament or the Femoral Head ligament, connecting to the articular head of the femur from the cup of the pelvis itself thereby gluing the femur directly into the hip socket while still permitting a good range of motion.

The stifle consists of the distal end of the femur, the patella, and the proximal end of the tibia and is the largest joint in the body. And, of course, there’re powerful ligaments lashing it together. These ligaments also help to form the Stay Apparatus (discussed in Part IV) that’s so important to understand. The Medial and Lateral Collateral ligaments and the Medial and Lateral Femoropatellar ligaments help to hold the joint steady while the MedialMiddle, and Lateral patellar ligaments lock the patella down onto the femur and play a large part in the Stay Apparatus. The Cruciate ligaments within the stifle itself stabilize the joint, inhibiting lateral overflexion.

Mechanically speaking, however, the stifle joint (like our knee joint) is cobbled together and is intrinsically more unstable than, say, the elbow joint. This is because the hindlimbs were the last to flip over forwards in our fishy ancestors unlike the forelimb, which was the first. This means that the joint surfaces of the stifle (like our knees) are imperfect and are jury-rigged together by ligaments. This is one of the reasons why our knee is so vulnerable to injury whereas our elbows aren't...and the same is true for the horse.

The lower legs have a complex network of ligaments that help to give the lower legs their shape and, for the most part, are clearly seen on the surface except for the deepest aspects. They’re all responsible for joint stabilization and support while also helping to dictate articular parameters. A couple of important ones to know are the Collateral Carpal ligaments, the Collateral Tarsal ligaments, the Long Plantar Ligament, the Collateral Sesamoidean ligamentsStraight Distal Sesamoidean ligament, and the Oblique Distal Sesamoidean ligament.


Skin is the largest and heaviest organ of the body with some very important and complex functions. For starters, it protects the inner systems from environmental injury, toxins, and microbes. It also helps to regulate body temperature through sweat and vasodilation. It contains tiny smooth muscles in the dermis, arrector pili muscles, that in contraction, can make the hair stand up, further aiding insulation or increase heat loss. The skin provides sensation with the outside world, too, communicating important stimuli to the brain. It inhibits dehydration by reducing the loss of essential fluids while its glands help to eliminate body toxins or release sexually stimulating pheromones. The skin also produces the essential Vitamin D in collusion with sunlight. As if that weren't enough, the skin has ample antigen fighting cells that present the first layer of attack against disease. To top it all off, it readily repairs itself. Quite an amazing thing, skin is! 

It has three primary layers: the epidermis, the dermis, and the subcutis. The outermost layer is the epidermis. It lacks blood vessels (avascular) and has many layers that vary in thickness and nature dependent on the different body area. It can also be divided into layers based on the cell types and shapes of the epithelial cells, specifically the keratinocytes, melanocytes, and Langerhans cells. Keratinoctyes are in the greatest percentage and they create keratin, a protein that forms the skin, hair, and horn. In this respect, the hair, chestnuts, ergots, and horn are really extensions of the epidermis. In the skin, the keratinoctyes are constantly replacing themselves, and the entire process by which these cells mature into their final state is called keratinization. Melanocytes live primarily in the basal cell layer and produce melanin, the primary skin pigment. They synthesize a key enzyme, tyrosinase, which is turned into an amino acid, tyrosine, that is then converted into melanin. The melanocytes then transfer the melanin to the young keratinocytes (the basal cells), which then disperse it via their replication. Langerhans cells create immune reactions in the skin to combat infection by processing antigens, particularly with allergic reactions.

The dermis constitutes most of the skin, being the thickest portion. It’s mostly made up of connective tissue comprised of about 90% collagen and 10% elastin. It also helps to provide nourishment, strength, and support to the epidermis. It has two layers, the superficial layer composed of loosely arranged collagen fibers, and the deep layer having dense and tightly webbed collagen fibers. Continuing through the epidermis, but originating in the dermis, the skin has specialized epithelial structures such as the hair follicles, apocrine sweat glands, and sebaceous glands. The sweat glands release their contents onto the hair shaft and, in the horse, are found over most of the body except most of the legs. They actively function to help regulate temperature through evaporation. The sebaceous glands produce sebum, a lubrication for the skin and hairshaft that also provides some measure of antimicrobial protection.

The innermost layer is the subcutis or hypodermis which is made primarily of fat cells and connective tissues comprised of collagen fibers. Nerves and vessels wander through this layer to reach the dermis.


Fat literally rusts; fat is simply oil that has oxidized into a more solid substance which then further oxidizes into wax. Accordingly, oily fat is new fat whereas wax is old, oxidized fat (and is therefore very difficult to get rid of...grrrr).

From carbohydrates and proteins, the liver makes fat and stores it within itself and elsewhere in the body, sometimes in the bubble fascia (where it’s known as cellulite). B vitamins are essential for the transport of fat from the liver. Bile produced by the liver is also important for the absorption of dietary fat as well as the removal of cholesterol and waste products in blood. 

Fat has many uses in the body, but the primary function is for energy storage. However, horses cannot usually metabolize fat for energy during high-speed exercise. Rather, fat can be used during other times for energy requirements. 


Blood is comprised of two cells, red and white, suspended in a fluid called plasma.

Red blood cells (erythrocytes) make blood the color red by their red pigment, hemoglobin. Unlike other cells in the body, they don’t have a nucleus. They look like fat, round pillows that have been mushed in the middle on both sides and contain about 60% water and 33% hemoglobin. Yet, they’re also able to change shape considerably to squeeze through tight areas. An average horse has about 3,200 million red blood cells per teaspoon of blood. Their purpose is to carry oxygen gas, which they gather from the lungs, with which hemoglobin has an affinity. It binds to the oxygen, creating oxyhemoglobin which the red blood cell then carries to the body. The tissues absorb the oxygen and the muscles use it to burn carbohydrates (sugars) to create energy. The red cells then return to the lungs and begin the process all over again. The red cells also transport carbon dioxide from the tissues back to the lungs, a waste product of burning carbohydrates. But they have a limited life span of only thirty days, with more being continually created in the bone marrow. The old ones are reabsorbed in the spleen and liver.

White blood cells (leucocytes) are outnumbered by their red counterparts by about 1000 to 1. Although there are five kinds of leucocytes, they’re all involved in the immune response attacking infection, foreign material, and other threats to the body.

Plasma is made mostly of water (about 93%) with certain proteins and mineral salts, and makes up normally about 40%-50% of the liquid we see as blood. The plasma also contains fibrinogen which is part of the clotting process. The proteins in plasma are also associated with the immune system as well as with enzymes, vitamins, and the endocrine system.

Horn and Hair

Horn entails those areas that have “cornified” or hardened to either a tough horn or nail-like consistency, or even a tough leather-like consistency though the process of keratinization. Such areas include the hoof capsule, sole, frog, chestnuts, hair, and ergots. These areas are trimable; the horn of the foot is either trimmed away by a farrier or sloughed off by natural wear. The chestnuts (also called “night eyes”) and ergots (also rarely called "hooks") can be peeled away or naturally drop off. Chestnuts are believed to be remnants of the carpal and tarsal foot pads (or even perhaps the remnant of the first digit) and ergots are thought to be leftovers of the metacarpal and metatarsal pads. 

Hair is secreted by living collagen cells within the hair follicle. So while the hair is essentially a dead structure, the collagen cells creating the hair shaft are living. Hair is grown then periodically shed. A hair shaft is grown, becomes dormant then is shed as a new shaft begins to grow underneath to push it out. The hair growth and shedding becomes synchronized to the seasons, most obvious in wild horses living in cold climates. The length of daylight seems to be the trigger for the growth and subsequent shedding of the winter coat. Hair is of great practical importance for the horse by being an efficient insulator and providing a level of skin protection from cuts, rubs, heat, sunburn, and chemical irritation. It’s also a source for keratinoctyes to help heal wounds.

Conclusion to Part II

These are just the bare-bone basics (pardon the pun) in regards to tissues. The nature of the animal's body is fascinating and ripe with discovery, so it's hoped this post will be an impetus to learn more about the physical make-up of our subject. Like us, the animal is comprised of a colony of cells, living cooperatively towards the same goals: life, survival, and reproduction. To know that there's living experience within living experience only makes life all the more of a miracle...and all the more of a miracle is the equine!

In Part IV then, we'll take the next step and explore some of the major mechanical and biological systems of the equine. The equine's body can be broken down into specific systems, but the major point to take away is that these systems always function holistically; there's no part of the equine body that functions independently. He truly is the sum of his parts.

So until next time...tissues! Gesundheitt!

"The creation of art is a continuous journey of self-discovery and learning the craft of illusion. Perfecting it, we only discover there's more to it than meets the eye."

~ Alfred Muma

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