Heads Above The Rest!; Exploring The Science and Art Of The Equine Head for Sculpting: Part 9

Introduction

Hey there! Welcome back to this 20–part series examining the equine head, anatomically, biologically, and artistically. So far we’ve covered a lot of ground, but we’ve got many more features to explore. But now that we've laid down a bit of a foundation, we can progress to muscles and other fleshy bits that are so important for our clay, too. Yet we shouldn't let the seeming simplicity of the equine head musculature fool us. It's really quite complicated even though it’s relatively sparse only because so much of it is interlaced, making many deep layers and superficial layers rather arbitrary distinctions. It's also capable of so much motion and expression, of even the subtlest of tweaks, that it can be a real challenge to portray well. There's also a lot to juggle and balance because so much is crammed into such a relatively small space. 

For these reasons then, the muscles are separated into multiple parts to keep posts to a manageable, chewable size. So without further adieu, let’s just get to it, shall we?…

 Muscles

Much of the skull is close to the surface while most of the cartilage is subcutaneous despite it all being interlaced with a network of delicate muscles, fascia, tendons, ligaments, and hide. Muscles on the head tend to be flat or strap–like, too, with fleshy muscle bellies reserved mostly for specific muscles such as the Buccinator and Masseter, on the sides of the head. Overall, the flesh of the head can be categorized into nine basic types, as follows:

• Orbital
• Eyelids
• Ear
• Upper face and mandible
• Muzzle and mid–face
• Nostrils
• Tongue
• Hyoid Apparatus
• Cutaneous muscles

Orbital Muscles

There are seven basic muscles of the orb—four straight muscles, two oblique muscles, and one retractor muscle. They’re small and encased in fascia connecting them to each other and to the lids. There also exist three muscles within the orb itself, which are tiny. That said, however, the movements of the orb within the socket aren’t as straight–forward as implied by these muscle attachments because, together, they produce rather complex movements. Also, the insertions of the rectus muscles aren’t at equal distances from the orb’s equator and the actions of the oblique muscles aren’t consistent with the longitudinal axis of the orb. In short, it’s not as simple as it seems! Nevertheless, the orbital muscles are, as follows:

• Obliquus dorsalis superior (also called the dorsal oblique): The narrowest and longest of the eye muscles, it makes a 90˚ turn at the trochlea attached to the front part of the orbit’s medial wall, the medial border of the optic foramen (which has a bursa at that point) to then continue to its insertion, dorsal on the orb, deep to the insertion of the rectus dorsalis. It rotates the orb on an inward axis, raising the front of the pupil in a spinning motion. It gets its blood supply from the external ophthalmic artery and its nerve supply from the trochlear nerve.
• Obliquus ventralis inferior (also called the ventral oblique): A wide, short muscle, it arises ventral to the fossa of the lacrimal sac to curve around the rectus ventralis then towards its insertion,  ventrolaterrally on the orb passing superficially to the rectus ventralis and partly beneath the rectus lateralis. It rotates the orb on an outward axis, lowering the front of the pupil in a spinning motion. Its blood supply is the external ophthalmic artery and its nerve supply is the oculomotor nerve.
• Rectus dorsalis (also called dorsal rectus): A band–like muscle that inserts by thin tendons just in front of the equator of the orb. It originates on the sphenoid bone next to the optic foramen via a common tendinous ring on the pterygoid crest and around the optic foramen with the rectus lateralis, rectus ventralis, and the rectus medialis to then insert on the dorsal surface of orb. It rotates the pupil rostral and up (upwards, on a horizontal axis). When all rectus muscles are contracting, they can retract the orb into the orbit. Blood is supplied by the external ophthalmic artery and nerve impulses by the oculomotor nerve.
• Rectus ventralis (also called ventral rectus): A band–like muscle that inserts by thin tendons just in front of the equator of the orb. Originating on the sphenoid bone next to the optic foramen via a common tendinous ring on the pterygoid crest and around the optic foramen with the rectus dorsalis, rectus lateralis, and rectus medialis, it inserts on the ventral surface of orb. It rotates the orb rostral and down (downwards, on a horizontal axis), and when all rectus muscles are contracting, they can retract the orb into the orbit. The external ophthalmic artery supplies blood and the oculomotor nerve provides nerve impulses.
• Rectus lateralis (also called lateral rectus): A band-like muscle that inserts by thin tendons just in front of the equator of the orb. This muscle originates on the sphenoid bone next to the optic foramen via a common tendinous ring on the pterygoid crest and around the optic foramen with the rectus lateralis, rectus ventralis, and the rectus medialis. It then inserts on the lateral rostral surface of orb. It rotates the pupil forwards (laterally, on a perpendicular axis). The external ophthalmic artery provides blood and the abducent nerve is the source of impulses.
• Rectus medialis (also called medial rectus): A band–like muscle that inserts by thin tendons just in front of the equator of the orb. Its origin is the sphenoid bone next to the optic foramen via a common tendinous ring on the pterygoid crest and around the optic foramen with the rectus dorsalis, rectus lateralis, and the rectus ventralis. It inserts on the medial surface of orb. It rotates orb rostral medially (backwards on a perpendicular axis). The external ophthalmic artery is its blood supply and its innervated by the oculomotor nerve.
• Retractor bulbi (also called the retractor oculi): It encloses the optic nerve and originates between the optic foramen and the orbital fissure and inserts on the orb, caudal to equator. It retracts orb into the orbit. The facial and palato-labial arteries supply blood and the oculomotor nerve and lateral portion of the abducent nerve supply nerve impulses.
• Cillary muscle (also called the ciliaris): A smooth muscle with circular fibers that suspend the lens within the orb. Its contraction or relaxation focuses the lens. However, it’s thought that the cillary muscles of the horse are poorly developed, having a limited ability to change the shape of the lens to view nearby objects well. Its origin is the scleral ring and it inserts on the lens capsule by tiny fibers. It works to reshape the lens for focusing. The facial and palato–labial arteries supply blood and the parasympthatic fibers from the short ciliary nerve provide impulses.
• Sphincter pupillæ: This is a smooth muscle within the iris, surrounding the pupil, mostly with concentric fibers. It originates as a circular course around the border of the pupil and acts to constrict the pupil. Blood is supplied by the facial and palato-labial arteries, and nerve impulses by the parasympathetic fibers from the oculomotor nerve.
• Dilator pupillæ: A smooth muscle within the iris with fibers that radiate outward. Its origin is a radial course around the border of the pupil and its action dilates pupil. Its blood supply is the facial and palato-labial arteries and its nerve impulses are supplied by its sympathetic fibers.

Muscles Of The Eye Lids

The eyelids are thin and delicate, and highly expressive. They can be pigmented (various shades of black, charcoal, brown, or even a pinkish, purply, or blueish brown or mocha), partially pigmented (“mottled”), or unpigmented (pink), depending on coat color or pattern. They’re divided into four basic layers—the skin, the eyelid muscles, the fibrous tarsal plate, and the innermost palpebral conjunctival layer. For simplicity, however, we’ll only deal with the muscles and the skin:

• Levator palpebræ superioris: A thin band of muscle about half an inch wide that runs above the rectus dorsalis, laying almost entirely within the orbit. It originates dorsal to the optic foramen along the pterygoid crest; it’s more narrow at its origin, and inserts on the upper eyelid by a thin, broad tendon. It raises the upper eyelid. It gets its blood supply from the Ophthalmic artery, and nerve supply from the Oculomotor nerve.
• Orbicularis palpebrarum (also called the Orbicularis oculi): A flat, elliptical sphincter muscle encircling the orbit and partially onto the orb. It’s upper portion is broader and better developed than the lower portion, and forms the shapes of the lids. Its primary attachment to the skull is through the skin of the lids and to the lacrimal bone and tubercle to insert on the eye lids, serving to open and close the eye. Blood is supplied by the facial, transverse, facial, supraorbital and infraorbital arteries. It gets its nerve impulses from the facial nerve.
• Corrugator supercilii (also called the levator anguli oculi medialis, or the levator muscle of the eye): A small, thin, triangular muscle. It originates on the frontal bone, and inserts by blending with the Obricularis palpebrarum in the upper anterior corner of the eye. It works to pull the Orbicularis palpebrarum upward, wrinkling the skin above the eye, but not affecting the shape of the upper lid itself. Blood is supplied by the facial, transverse, facial, supraorbital and infraorbital arteries and nerve impulses from the facial nerve
• Malaris (also called the lachrymal, or the depressor muscle of the eye): A flat muscle that originates on the masseteric ridge and inserts by blending with the Obicularis palpebrarum. It works to pull down the lower eyelid, enlarging the eye by rounding the lower rim. This muscle is variable among individuals and may account for some subtle differences in expression between individuals or breeds. Blood is supplied by the facial, transverse, facial, supraorbital and infraorbital arteries, and nerve impulses come from the facial nerve.

Muscles Of The Ear

Finding reliable anatomical references for the equine ear muscles is surprisingly difficult, largely being treated lightly or even outright ignored. They’re also often referenced differently and muscles may be included with others, creating rather confusing interpretations. The best reference, however, can be found in the book, ANATOMY OF THE DOMESTIC ANIMALS by Sisson and Grossman, 4th Edition. That in mind, ear muscles can be described, as follows:

• Scutularis: A thin sheet of subcutaneous membranous muscle overlaying the Temporalis muscle. It originates on the zygomatic arch and the frontal and parietal crests to insert on the scutiform cartilage. It can be described in three parts (sometimes it’s described as three separate muscles in other references). Yet, in total, this muscle stabilizes the scutiform cartilage and rotates the ear forwards to varying degrees depending on the characteristics of each branch or expression. These three parts are: (1) Frontoscutularis (also written as Fronto-scutularis pars temporalis, or Fronto-scutularis, Attollens anticus, Scutularis, or the Temporoauricularis, with a frontal and temporal portion): Each portion is a flat, band-like muscle. It arises from the zygomatic arch and the frontal crest, and inserts on the lateral (from the zygomatic arch or temporal portion) and anterior borders (from the frontal crest or frontal portion) of the scutiform cartilage, (2) Interscutularis (also called Interscutular muscle, or the Attollens maximus, or simply the Scutularis): A broad, band-like flat muscle that fans slightly forwards down the median. The two bilateral parts of this muscle converge a bit down the median, giving them impression that the muscle goes between both scutiform cartilages. It arises from the parietal crest (the crista sagittalis) and inserts on the medial rim of the scutiform cartilage, (3) Cervicoscutularis (also written Cervico-scutularis): Not clearly defined from the Interscutularis (and often regarded as the same muscle in some references). Like the Interscutularis, it’s also a broad, band-like flat muscle that fans slightly down the median, but backwards. It arises from the nuchal crest and ridge of the occipital bone, and inserts on the medial rim of the scutiform cartilage.
• Zygomaticoauricularis (also written Zygomatico-auricularis, or the External adductor of the auricle): A flat, band-like muscle that originates on the zygomatic arch and the parotid fascia, and inserts on the outer face of the conchal cartilage at its base, but partially under and partially above the insertion of the Parietoauricularis. It pulls the ear forward.
• Scutuloauricularis superficialis inferior (also written Scutulo-auricularis superficialis inferior, or the Inferior adductor of the auricle): A flat, band-like muscle that originates on the lateral aspect of the scutiform cartilage to insert on the base of the conchal cartilage beneath of the insertion of the Zygomaticoauricularis. It pulls the ear forward.
• Scutuloauricularis superficialis medius (also written Scutulo-auricularis superficialis medius or referred to as the Superior adductor of the auricle when interpreted as part of the Scutuloauricularis superficialis superior): A wide, flat band of muscle that originates on the posterior of the deep surface of the scutiform cartilage, and inserts on the dorsal aspect of the conchal cartilage bulb. It receives a slip from the Cervicoscutularis. It pulls the ear forward and erected upright.
• Scutuloauricularis superficialis superior (also written Scutulo-auricularis superficialis superior, the Scutuloauricularis profundi, or the Superior adductor of the auricle when interpreted as part of the Scutuloauricularis superficialis medius): A thin, flat band of muscle that appears as a prolongation of the Interscutularis (and sometimes considered part of that muscle in some references). It arises from the parietal crest (or from the Interscutularis, depending on the reference used) to pass over the medial rim of the scutiform cartilage to insert on the back top portion of the conchal cartilage’s bulb. It pulls the ear forward and erected upright.
• Scutuloauricularis superficialis accessorius (also written Scutulo-auricularis superficialis accessorius): A thin, narrow band of muscle, partially covered by the Scutuloauricularis superficialis superior. It originates on the posterior bulge and surrounding superficial areas of the scutiform cartilage, and inserts on the dorsal convex surface of the conchal cartilage, just behind the scutuloauricularis superficialis superior. It pulls the ear upward and rotates the ear backward.
• Parietoauricularis (also written Parieto-auricularis, or the Levator of the auricle): A flat, triangular muscle arising by a broad fan at the parietal crest under the Cervicoscutularis, passing a little backward and inserting by a narrow band on the lower aspect of the convex surface of the chonchal cartilage bulb, under the Cervicoauricularis superficialis. It rotates the ear backward.
• Cervicoauricularis superficialis (also written as Cervico-auricularis superficialis): A thin, triangular muscle that originates by a broad fan at the nuchal crest and the neighboring aspect of the nuchal ligament, becoming narrower as to runs to its insertion on the medial aspect of the convex surface of the conchal cartilage. It pulls the ear upright and rotated backwards.
• Cervicoauricularis profundus major (also called the Cervicoauricular muscle, the Caudal auricular, or as the Cervico-auricularis profundus major): A thin sheet of muscle that originates on the nuchal ligament, partially under and behind the Cervicoauricularis superficialis. It inserts on the posterior side aspect of the conchal cartilage bulb, partially under the Parotidoauricularis. It pulls the ear backward and rotated backwards.
• Cervicoauricularis profundus minor: A thin sheet of muscle that originates under the Cervicoauricularis profundus major, passing downward to insert on the lowest aspect of the conchal cartilage “bulb,” partially under the parotid gland. It pulls the ear downward and rotated backwards.
• Parotidoauricularis (also written Parotido-auricularis, the Depressor of the auricle, the Ventral auricular, or the Parotidoauricular muscle): A broad, band-like thin muscle that lays over the parotid gland behind the ramus, becoming a bit thicker and more narrow as it runs to the ear. It originates on the fascia of the parotid gland and inserts on the conchal cartilage, just below the “V” made by the two rims. It pulls the ear laterally and downwards.
• Scutuloauricularis profundus major (also written Scutulo-auricularis profundus major, the Abductor of the auricle, or the Scuto-auricularis externus): A flat, thin muscle about one inch wide that arises on the deep surface of the scutiform cartilage and inserts on the most prominent aspect of the conchal cartilage bulb. It pulls and rotates the ear forward.
• Scutuloauricularis profundus minor (also written Scutulo-auricularis profundus minor): A flat, thin muscle about one inch long that originates on the posterior aspect of the deep surface of the scutiform cartilage and the Cervicoscutularis, passing backwards and downwards. It inserts on the base of the concha cartilage, under the Scutuloauricularis profundus major. It pulls and rotates the ear forward.
• Tragicus (also called the Mastoido-auricularis): A small, thin, basilar muscle embedded in the fat that pads the conchal cartilage in the skull. It arises on the temporal bone behind the external acoustic process (or external auditory meatus) and over the annular cartilage, passing upwards to its insertion. It inserts on the lower aspect of the anterior surface of the conchal cartilage. It pulls the conchal cartilage towards the external auditory meatus.
• Antitragicus: A tiny, thin muscle comprised of just a few muscle bundles, partially blended with the Parotidoauricularis. It arises from the bulb of the conchal cartilage behind the “V,” and inserts, flowing into the depression made by the “V,” to blend with the Parotidoauricularis. It has no discernible action by itself.
• Helicis: A tiny, thin muscle that arises from the anterior rim of the conchal cartilage, and inserts, flowing into the depression made by the “V,” to blend with the Parotidoauricularis. It has no discernible action by itself.
• Verticalis auriculæ: A thin sheet of muscular and tendinous fibers that arises from the bulb of the conchal cartilage to insert on the convex dorsal surface of the conchal cartilage. It has no discernible action by itself.
• Styloauricularis: (?) Helps to pull down the ear. Unclear presence in multiple references.

Conclusion To Part 9

That’s a lot to mull over, isn’t it? Oh, but wait—there’s more! You’re not gettin' off that easy! In many ways, the head is perhaps the most difficult part of the equine's anatomy to truly understand and portray. There's just so much, in such a comparatively small area. Because the head has to do so much with so little, too, everything seems to serve multiple purposes which only adds to the complexity of their structure and relationships. To truly understand his head also means we come to truly understand what it means to be equine as well, something that informs the rest of what we do. The head has much to teach us, and if we remain open, we'll absorb all the lessons it offers. 

Approaching the head with some measure of humility is a good idea, too. It's a magnificent and utterly unique example of organic engineering, and we should respect its Biologic. It has purpose, it has consequences, and it has context—it's not just something we like to look at. In Part 10 then we’ll continue with facial muscling, so until next time…just muscle through it!

“My work is not repetition. It is an exploration.” ~ Guido Molinari