Welcome back to this series discussing the equine head from an anatomical, evolutionary, and artistic point of view. We’ve learned quite a bit so far, but we have more to go—there really is a lot to something that seems so simple, isn’t there? Biology is a complicated, messy thing, and everything is symbiotic and interdependent, so to understand one feature means we need to understand all the others. This is particularly true of the equine which evolution has shaped into what’s purely functional with great economy of form and function. In few other animals is so much done with so little. And it’s in the equine head where we see this efficiency so clearly and profoundly expressed. So let’s continue with our exploration with the last of his fascial muscles…
Muscles Of The Tongue
The tongue is the largest muscle in the horse’s head, and while myriad little muscles and fleshy bits related to it are present, we’ll only deal with the primary muscles, as follows:
- Lingualis proprius: The tongue itself, the root (radix linguæ) is attached to the Hyoids, soft palate, and the pharynx. Only the upper portion is free, and the root and lower portion are attached to the floor of the mandible. Comprised of longitudinal, transverse, and perpendicular muscle bundles, it’s the largest muscle in the head. Its simultaneous stiffening of its muscle bundles stiffens the tongue. Blood is supplied by the lingual artery and sub-lingual branches of the external maxilllary artery, and nerve impulses are supplied by the lingual, glosso-pharyngeal, and hypoglossal nerves.
- Styloglossus (also can be written Stylo-glossus): A long, thin muscle that lays along the lateral portions of the tongue. It originates on the lateral surface of the stylohyoid bone, near their articulation the keratohyoid bones, and inserts near the tip of the tongue, blending with its twin from the opposite side. Its dual action pulls the tongue up and back while its single action pulls the tongue to the side and up. The lingual artery supplies blood and the hypoglossal nerve provides nerve impulses.
- Hyoglossus (also written Hypo-glossus): A wide, flat muscle that’s a bit thicker than the Styloglossus. It lays on the lateral portions of the root and body of the tongue, partly covering the Styloglossus. Its fibers pass obliquely upwards and forwards, towards the median plane of the dorsal tongue. Its origin is the lingual process of the basihyoid bone, and the stylohyoid and thyrohyoid bones, and its insertion is the muscle body of the tongue; most fibers are near the median dorsal plane. Its dual action pulls the tongue down and back while its single action pulls the tongue to one side, down and back. Blood is supplied by the lingual artery, and nerve impulses from the hypoglossal nerve.
- Genioglossus (also written Genio-glossus): A fan-shaped muscle, laying parallel to the median plane of the tongue. Its origin is the medial surface of the mandible, just behind the symphysis; some fibers fan to the tip, some to the dorsal tongue and some to the root, and some even pass to the keratohyoid bones. It inserts on the tip and root of the tongue, fanning sagittally into it. Its dual action depresses the tongue and helps to form the dorsal groove when both muscles act. However, it also pulls the tongue forwards or backwards and down, depending which fibers are activated. Its single action moves the tongue to one side, either forwards or backwards. The lingual artery supplies blood and the hypoglossal nerve provides nerve impulses.
Muscles Of The Hyoid Apparatus
There are many muscles involved with swallowing, but the primary ones that activate the Hyoids are, as follows:
- Digastricus (already described)
- Mylohyoideus (also written mylo-hyoideus): With its twin, they form a sling to support the tongue within the free span of the mandible. It originates on the inner alveolar ventral surface of the mandible, and inserts by a fibrous median raphe to the symphysis of the basihyoid and to the lingual process on the basihyoid. It raises the floor of the mouth, tongue and Hyoids. The sublingual artery supplies blood and the mylohyoid nerve (from the mandibular nerve) supplies nerve impulses.
- Stylohyoideus (also written Stylo-hyoideus): A slender, fusiform muscle running at an almost parallel angle to the stylohyoid bone, it originates on the muscular process of the stylohyoid bone, and inserts on the thyrohyoid bone with its tendon encircling the round tendon of the Digastricus. It works to draw the tongue, larynx, and Hyoids upwards and backwards. Blood is supplied by the external carotid artery, and nerve supply from the facial nerve (from the stylo-hyoid branch).
- Occipitohyoideus (also written Occipito-hyoideus): A small triangular muscle, it lays between the top of the stylohyoid bone and the paramastoid process. It originates on the paramastoid process of the occipital bone, and inserts on the back and lower rim of the stylohyoid bone. It pulls the Hyoids backwards. Blood is supplied by the occipital artery, and nerve impulses form the facial nerve.
- Geniohyoideus (also written Genio-hyoideus): A long round muscle, it lays under the tongue next to its twin. It arises from a short tendon and its belly is comprised of long bundles of parallel muscle fibers. It helps to form the muscular basis of the oral cavity. Its origin is the medial surface of the ramus, from a small depression near the symphysis, and its insertion is the lingual process of the basihyoid bone. It pulls the tongue, larynx and Hyoids forwards. The sublingual artery is its blood supply and the hypoglossal nerve provides nerve impulses.
- Keratohyoideus (or also called Ceratoyhoid muscle, Ceratohyoideus, or also Kerato-hyoideus): Another small triangular muscle laying between the Keratohyoids and the Thyrohyoids. It originates on the back of the keratohyoid bones, the rostral border of the thyrohyoid bone, and the surrounding portion of the ventral rim of the stylohyoid bone. It inserts on the dorsal edges of the thyrohyoid bones. In action, it raises the thyrohyoids and the larynx. Blood is supplied by the lingual artery, and nerve supply from the glosso-pharyngeal nerve.
- Hyoideus transverses: A small singular muscle that attaches between the keratohyoids. When contracted, it raises the root of the tongue; when relaxed, it remains slack. Blood is supplied by the lingual artery, and nerve impulses by the glosso-pharyngeal nerve.
- Sternohyoideus: It’s divided mid–neck by a transverse tendinous intersection that separates it into cranial and caudal parts. It originates on the manubrium of the sternum, and inserts on the linguinal process of the basihyoid bone. It retracts and depresses the Hyoids and the base of the tongue. The common carotid artery supplies blood and the ventral branch of cervical nerve, 1 and 2 are its nerve supply.
- Sternothyrohyoideus (also written Sternothyro-hyoideus ): A long slender digastric muscle running below the trachea; sometimes regarded as a muscle of the neck. Sometimes its rostral continuation is considered a separate muscle, the Thyrohyoideus. Like the Sternohyoideus, it’s also divided mid–neck by a transverse tendinous intersection that separates it into cranial and caudal parts. It originates on the manubrium of the sternum, and inserts on the basihyoid (its lingual process) and the thyroid cartilage of the larynx. In action, it depresses and pulls backwards the Hyoids, tongue and larynx; when the depressor muscles of the tongue are contracted, the Sternothyrohyoideus may also fix the Hyoids for the sucking mechanism. Blood is supplied by the common carotid artery, and nerve supply from the ventral branch of cervical nerve, 1 and 2.
- Omohyoideus (or Subscapula-hyoidens) : A thin fleshy muscle, it forms the floor of the jugular groove of the neck. It originates on the subscapular fascia near the shoulder joint, and inserts on the lingual process of the basihyoid bone. It pulls back the Hyoids and tongue. The common carotid artery and superficial cervical artery supply blood and the ventral branch of the first cervical nerve provides nerve impulses.
The head has important cutaneous muscles, or bundles of tiny muscles embedded in the superficial fascia. Because these cutaneous muscles lay within the fascia of the skin, they have little or no attachment to the skeleton. These facial cutaneous muscles run from the larynx to the mouth and move the facial skin while helping to retract the mouth. They become very thin around the orifices and blend with the periosteum on the nasal and frontal bones. Specifically, the head has two primary cutaneous muscles:
- The cutaneous fasciei: Thin and typically incomplete muscular layer, it sweeps over the mandibular space and the masseter muscle. One branch runs to the angle of the mouth and blends with the orbicularis oris. This part becomes…
- The cutaneous labiorum (also called the retractor anguli oris): An extension of the cutaneous fasciei and helps to pull up the angle of the mouth.
Conclusion To Part 11
Phew! That was a lot to wade through, wasn’t it? Extra points for sticking around! Now it may seem like overkill to have gone into such detail, but the truth is we need to know just how complex the equine head truly is if we’re going to fully appreciate it. That is to say, it's quite different to look at the horse's head with "conventional" eyes than it is to perceive it with penetrating ones. The typical would have us see the head with a more superficial perspective, focusing more on breed type and "beauty" than anything else. This is fine, but only if we have a biological perspective, too. Indeed, it's this view of the head that gifts us with context, of the organic parameters that determine its shape and function. If our choices aren't seated in biological context first then, we risk not only misinterpreting breed type, but distorting it into nonviable forms. We also may miss subtleties of expression and gesture intrinsic to equine communication and personality, or even create errors when it comes to gender, age, and species' differences. So much is going on in any given second, we need to be hyper–aware of all these little tweaks that will influence its nature and features. Truly, being able to See the equine head with a diagnostic view can help us in so many ways beyond simply capturing its innate beauty. So until next time…heads up for Part 12!
“I obliged myself to explore where I might otherwise not have. And that’s what ‘mind-flexing’ is all about – making those brain-muscles work so that you feel empowered to pursue your own vision.” ~ Tony Smibert