MUSCLES OF THE HEAD

Muscles of the Head

Overview

The muscles of the head are divided into nine groups based on their embryonic origin and innervation:

(1) Muscles of facial expression: Innervated by the facial nerves.
(2) Masticatory musculature: Primarily innervated by the mandibular nerves from the trigeminal nerves.
(3) Extrinsic eye musculature: Innervated by the oculomotor, trochlear, and abducens nerves.
(4) Tongue musculature: Supplied by the hypoglossal nerves.
(5) Muscles of the pharynx: Innervated by the glossopharyngeal and vagus nerves.
(6) Soft palate muscles: Innervated by the trigeminal, glossopharyngeal, and vagal nerves.
(7) Laryngeal musculature: Supplied by the accessory and vagus nerves.
(8) Hyoid muscles: Innervated by the trigeminal, hypoglossal, and cranial cervical nerves.
(9) Cervical vertebral muscles: Insert on the skull and are innervated by cervical nerves.

I. Muscles of Facial Expression

Superficial Muscles

Originate from three layers of the primitive sphincter colli.
Include:
- m. sphincter colli superficialis
- platysma
- m. sphincter colli profundus

m. sphincter colli superficialis

Best developed in the laryngeal region, deep to the skin.
Transverse fibers span the ventral borders of the platysma muscles at the junction of the head and neck.
Occasionally reaches the thorax, radiates over the shoulder joint, or blends with the cervical part of the platysma.

Platysma

Well-developed muscle sheet.
Originates from the mid-dorsal tendinous raphe of the neck and the skin.
Two separate layers of origin fuse near the midline.
Extends over the parotid and masseter regions to the cheek and commissure of the lips, where it radiates into the m. orbicularis oris.
In the lips, it is designated as the m. cutaneus facei.
The portion in the neck is known as m. cutaneous colli.
At the ventral midline, these bilateral cutaneous muscles approach each other and meet caudal to a transverse plane through the commissures of the lips (when well developed).
Covers large portions of the m. sphincter colli profundus.
The dorsal border is united with the underlying sphincter colli profundus by many fiber bundles.
The ventral border has a distinct boundary and rarely has defects.
Action: To draw the commissure of the lips caudally.
Innervation: Buccal branches and the caudal auricular nerve from the facial nerve (rami buccales et n. auricularis caudalis, n. facialis).

m. sphincter coli profundus

Consists of a few thin muscle fascicles.
Extends dorsoventrally from the base of the ear, lateral to the masseter muscle and parotid gland.
Covered by the platysma.
Extends across the ventral median plane to fuse with the fascicles of the same muscle on the opposite side.
A few fascicles extend dorsoventrally from the orbicularis oculi deep to the platysma.

Deep Muscles of the Lip and Nose

Include:
- orbicularis oris
- zygomaticus
- superior and inferior incisivus
- levator labii superioris
- caninus
- buccinator
- mentalis
- levator nasolabialis
Innervation: Facial nerve; most by dorsal or ventral buccal branches of the facial nerve (rami buccales n. facialis).

m. orbicularis oris

The principal component of the lips.
Extends from the commissural region into the lips near their free borders.
Interrupted in the rostral median segment of both lips.
Lies between the skin and the mucosa.
Other muscles of the lips and cheeks (platysma; mm. buccinator, zygomaticus, and levator nasolabialis) enter the m. orbicularis oris caudally, blending with each other.
The m. incisivus is also attached to it rostrally.
The portion in the superior lip is the thicker component.
Separate fibers extend from it to the external naris.
Action: Closes the lips and is a pressor of the labial glands; medial bundles pull the entire nose ventrally (sniffing), lateral bundles increase the diameter of the external nares. In strong contractions, both medial and lateral fiber bundles function to dilate the external nares.

m. zygomaticus

Arises from the scutiform cartilage.
Straplike, long muscle extends from the rostral angle of the scutiform cartilage to the edge of the superior lip and cheek, sinking into the orbicularis oris after crossing deep to the rostral fibers of the sphincter colli profundus.
Its rostral portion is deep and bears no relationship to the platysma.
Proximally, it is distinctly separated from the m. frontalis and is covered by the skin.
Action: To fix the angle of the mouth and draw it caudally, or to fix and draw the scutiform cartilage rostrally.

mm. incisivus superioris et inferioris

Lie deep to the orbicularis oris.
Two thin muscles not clearly defined from the orbicularis and buccinator.
Arise on the alveolar borders of the incisive bone and mandible as far as the corner incisor teeth.
Situated immediately deep to the mucosa of the lips.
Action: The m. incisivus superioris raises the superior lip, while the m. incisivus inferioris depresses the inferior lip.

m. levator labii superioris

A flat muscle that lies deep to the apical end of the levator nasolabialis on the maxilla and incisive bone.
Arises from the maxillary bone caudoventral to the infraorbital foramen and courses rostrally along the dorsal border of the caninus muscle.
Fibers of insertion spread out as they enter the nasal ala and the superior lip.

m. caninus

Immediately ventral to the levator labii superioris, extends rostrally deep to the labial end of the levator nasolabialis.
Terminates rostrally in the superior lip.
Action: To increase the diameter of the external naris and to lift the apical portion of the superior lip.

m. buccinator

Developed from the deep part of the orbicularis oris.
A thick, flat, wide muscle that forms the foundation of the cheek.
Composed of two portions, which extend caudally from the labial commissure:
- buccalis part (pars buccalis), formerly called pars dorsalis
- molar part (pars molaris)
The buccal part is somewhat larger. It arises from the maxilla dorsally and the mandible ventrally deep to the orbicularis oris.
It consists primarily of longitudinal fibers where it is located in both lips.
Caudally in the cheek, the dorsal and ventral portions meet in a raphe that extends caudally from the commissure of the lips.
The molar part is deep to the buccal part and consists of longitudinal fibers that arise from the ramus of the mandible and course rostrally to fill the cheek and blend with the buccal part as well as the orbicularis oris.
The caudal portion of the molar part is overlapped by the masseter muscle.
Action: To return food from the vestibule to the masticatory surface of the teeth.

m. mentalis

Arises from the alveolar border and body of the mandible near the third incisor.
Fibers unite with those of the opposite side and radiate into the inferior lip, forming a prominent, fat-infiltrated muscle.
Action: To stiffen the inferior lip in the apical region.

m. levator nasolabialis

A flat, thin, and broad muscle lying immediately deep to the skin on the lateral surface of the nasal and maxillary bones.
Arises in the frontal region between the orbits from the nasofrontal fascia, the medial palpebral ligament, and the maxillary bone.
Occasionally, a few additional fibers come from the lacrimal bone.
Spreading out, it proceeds to the nose and superior lip to insert deep to the orbicularis oris.
The caudal portion inserts on the buccinator.
The apical, larger portion passes deep to the orbicularis to end near the edge of the lip.
The most dorsal and rostral fibers interdigitate with the fibers of the levator labii superioris to attach to the external naris.
Action: To increase the diameter of the naris and lift the apical portion of the superior lip.
Innervation: Auriculopalpebral nerve from the facial nerve (n. auriculopalpebralis, n. facialis).

Deep Muscles of the Eyelids, Forehead, and Ears

Include:
- orbicularis oculi
- retractor anguli oculi lateralis
- levator anguli oculi medialis
- levator palpebrae superioris
- orbitalis (ocular smooth muscles)
- occipitalis
- multiple ear muscles
Innervation: All by the facial nerve except the levator palpabrae superioris (oculomotor nerve) and the orbitalis (sympathetic nerves).

m. orbicularis oculi

Surrounds the palpebral fissure.
Portions adjacent to the borders of the lids extend from the medial palpebral ligament dorsal to the superior lid, around the lateral commissure of the lids, and along the inferior lid back to the ligament.
Thus in the dog, this muscle, which originally was divided into dorsal and ventral portions, has become one.
Action: To close the palpebral fissure.

m. retractor anguli oculi lateralis

Arises beside the m. frontalis from the temporal fascia.
Extends horizontally to the lateral palpebral angle, crossing the orbicularis oculi before sinking into its fibers.
Action: To draw the lateral palpebral angle caudally.

m. levator anguli oculi medialis

A small, thick muscle strand that arises from the median line on the frontal bone from the nasofrontal fascia.
Extends dorsal to the orbicularis oculi of the medial half of the superior eyelid.
Action: To elevate the superior eyelid, especially its nasal portion, and erect the hairs of the eyebrow.

m. levator palpebrae superioris

The main retractor of the superior eyelid.
Arises dorsal to the optic canal between the dorsal rectus and the dorsal oblique muscles.
Courses deep to the periorbita and superficial to the extraocular muscles in reaching the superior eyelid.
Inserts in the superior eyelid by means of a wide, flat tendon that passes between the fascicles of the m. orbicularis oculi.
Action: To lift the superior eyelid.
Innervation: N. oculomotorius.

m. orbitales

Smooth muscles associated with the eyeball, orbit, and lids.
Extraocular components include smooth muscle fascicles in the superior, inferior, and third eyelids as well as the periorbita.
Several of these, including the ventral and dorsal palpebral muscles, have been referred to in the past as muscles of Müller.
In the dog, a delicate fan of muscle fibers arises from the trochlear cartilage and inserts in the superior lid.
These fibers are nearly continuous at their insertion with the edge of the m. levator palpebrae superioris.
Action: Retract the eyelids and protrude the eyeball.
Innervation: Sympathetic postganglionic axons primarily within the branches of the ophthalmic nerve from V.

m. occipitalis

Lies superficial to the occipital and parietal bones.
From the external sagittal crest, its fibers turn rostrally in bilaterally symmetric arches, forming an unpaired, oval, thin membranous muscle.
Can be followed a short distance rostrally deep to the caudal portion of the m. interscutularis.
On the frontal bone, they spread out into the nasofrontal fascia.
Action: To tense the nasofrontal fascia.

Muscles of the External Ear

Organized into five groups:
- Four extrinsic muscle groups: rostral, dorsal, caudal, and ventral.
- One intrinsic group.
The names of these muscles have been subjected to extensive revisions.

Intrinsic ear muscles

Include the helicis, helicis minor, tragicus, transverse auricular, and oblique auricular.
The mm. helicis, helicis minor, and tragicus lie together in one muscle complex that bridges the space between the superimposed conchal cartilage edges at the opening of the conchal cavity.
The concha auriculae is the funnel shaped proximal part of the auricle.
This muscle aggregate passes from the deep surface of the lateral crus of the helix to the tragus.
In certain cases, all of these muscles are independent.
The m. helicis arises from the external surface of the tragus, the m. helicis minor from the tragus and the conchal canal, and the m. tragicus from the external surface of the concha.
Action: To narrow the entrance to the conchal canal and thus make the concha rigid.

Rostral extrinsic ear muscles

Include the superficial scutuloauricularis, deep scutuloauricularis, frontalis, frontoscutularis, and zygomaticoauricularis.

m. scutuloauricularis superficialis

The dorsal medial rotator of the ear.
Consists of two short broad bundles that arise from the lateral border of the scutiform cartilage and attach to the concha and the lateral crus of the helix.
Separates dorsocaudally from the m. frontalis, with which it is always partly united.
Courses in a fold of skin to the medial border of the concha.
Action: To turn the conchal opening rostrally and medially.

m. scutuloauricularis profundus

The large rotator of the concha.
Completely separated from the m. frontalis.
Lies deep to the scutiform cartilage and arises on its deep surface to extend to the concha adjacent to the m. temporalis.
The muscle has an almost sagittal course.
Action: To turn the conchal fissure caudally.

m. frontalis

A thin muscle that lies on the temporalis.
Arises rostral to the rostral border of the scutiform cartilage, by means of a fascial leaf, and extends to the forehead and toward the superior eyelid.
Spans across the midline to unite with the opposite muscle rostral to the m. interscutularis.
Rostrally, it joins the nasofrontal fascia by which it attaches to the zygomatic process.
From the external ear cartilage a considerable number of muscle strands of the m. scutuloauricularis superficialis extend over the scutiform cartilage into the frontalis.
Action: To fix and pull the scutiform cartilage rostrally.

m. frontoscutularis

Has formerly been considered part of the frontalis muscle, which is adjacent to its medial border.
Arises from the rostral part of the scutiform cartilage and courses rostrally to terminate on the frontal bone and medial palpebral ligament medial to the medial portion of the orbicularis oculi and lateral to the levator anguli oculi medialis.

m. zygomaticoauriculari

The medial rotator that arises as a rather broad muscle from the tendinous leaf lying rostral to the scutiform cartilage.
Continuous rostrally with the ventral portion of the frontalis and is deep to the sphincter colli profundus.
Caudally, it extends ventrally to terminate on the basal portion of the tragus.
Action: To turn the auricular concha rostrally.

Dorsal extrinsic ear muscles

Include the interscutularis, parietoscutularis, and parietoauricularis.

m. interscutularis

A thin muscle extending from one scutiform cartilage to the other, without attaching to the cranial bones.
Has developed from the fusion of bilateral portions.
The origin is from the entire dorsomedial border of the scutiform cartilage.
The caudal portion of the muscle has a distinct border and covers the m. occipitalis and the m. cervicoscutularis, both of which blend with the interscutularis.
Rostrally it has no distinct border and encroaches upon the m. frontalis.
Action: Fixation of the scutiform cartilage.

m. parietoscutularis

Only exceptionally an independent muscle.
Arises from the interparietal portion of the external sagittal crest and inserts on the caudal border of the scutiform cartilage, which is completely covered by the superficial layer of this muscle complex.
Action: With other scutular muscles, it aids in fixation of the scutiform cartilage.

m. parietoauricularis

Only seldom completely isolated.
Indeed, it belongs to the deep layer but usually fuses with the m. parietoscutularis, which becomes separate only near the scutiform cartilage.
In its entire course, it is covered by the superficial layer of the caudal auricular musculature.
Arises from the interparietal segment of the external sagittal crest and goes directly to the dorsum of the concha, where it attaches deep to the caudal terminal branch of the m. cervicoauricularis superficialis basal to its insertion.
Action: To raise the concha.

Caudal extrinsic ear muscles

Include the cervicoscutularis, cervicoauricularis superficialis, cervicoauricularis medius, and cervicoauricularis profundus.

m. cervicoscutularis

A narrow, intermediate portion of the muscle complex that is not clearly defined.
Deep to the interscutularis and passes to the caudal border and the caudomedial angle of the scutiform cartilage.
United with the deep surface of the interscutularis by means of a few fibers.
Action: To draw the scutiform cartilage ventrally and caudally or fix it when the scutiform cartilage is drawn rostrally at the same time.

m. cervicoauricularis superficialis

Arises from the cervical midline and the external occipital protuberance.
As a broad muscle mass, it passes to the concha and ends by two branches on the dorsum of the ear.
The rostral branch is made wider by fibers from the lateral border of the scutiform cartilage.
These correspond to the m. scutuloauricularis superficialis or short levator, of other animals.
The caudal branch covers the auricular end of the parietoauricularis.
Action: To raise the concha.

m. cervicoauricularis medius

A thick, relatively wide muscle that, covered partly by the cervicoauricularis superficialis, arises on the external sagittal crest, the external occipital protuberance, and the neighboring attachment of the nuchal ligament.
Extends to the base of the concha and finally ends on the root of the lateral conchal border (antitragus), where it lies next to the insertion of the parotidoauricularis.
This muscle covers a portion of the origin of the parietoauricularis, the greater part of the cervicoauricularis profundus and the m. temporalis of that region.
Action: To turn the conchal fissure laterally and caudally.

m. cervicoauricularis profundus

A division of the deep layer of the caudoauricular musculature.
At its origin, it is rather variable in that it can be divided into two to five clearly defined muscle bundles. Of these, the caudal one usually comes from the external occipital protuberance, whereas the other portions are more or less shortened and arise by an aponeurosis from the m. temporalis.
Covered by the long lateral rotator, the muscle runs to the extended lateral conchal border.
Action: To turn the concha laterally and caudally.

Ventral extrinsic ear muscles

Composed of the parotidoauricularis muscle and styloauricularis.

m. parotidoauricularis

Arises caudal to the laryngeal region, on or near the midline, where it blends with the cervical fascia.
As a well-defined band, it runs obliquely dorsally toward the concha, crossing the mandibular and parotid glands in its course.
The muscle is almost completely covered by the platysma and inserts on the antitragus.
Action: To depress the ear.

m. styloauricularis

A muscle of the concha.
A long, narrow muscle that also bears the name m. tragicus lateralis in descriptive nomenclature.
Arises tendinously in the niche between the angular and the condyloid processes of the mandible and extends dorsally to the helix covered by the parotid salivary gland.
In its course, it passes over the root of the zygomatic process of the temporal bone, extends along the rostral side of the concha, and ends opposite the scutuloauricularis profundus.
This muscle may undergo great reduction, and in extreme cases may be represented only by tendinous remains. Often it is connected directly with the m. helicis, whose innervation it shares.

Muscles of the Middle Ear

The m. stapedius was originally associated with the hyomandibular bone of the primitive mandibular joint. During evolution the hyomandibular became the stapes and with its associated muscle was incorporated into the middle ear. This muscle, the m. stapedius, innervated by the facial nerve.
The m. tensor tympani, innervated by the trigeminal nerve.
The m. tensor tympani contains a unique myosin, referred to as 2M, that is not found in the adjacent m., supporting an independent origin of these two muscles.
The tensor tympani develops from branchial arch 1 with its trigeminal nerve innervation and the stapedius muscle develops from branchial arch 2 with its facial nerve innervation. The 2M myosin is also found in the trigeminal nerve innervated muscles of mastication derived from branchial arch 1.

II. Muscles of Mastication

Include the masseter, temporal, lateral and medial pterygoid, and the digastricus.
All are innervated by the mandibular nerve from the trigeminal except for the caudal portion of the digastricus, which is innervated by the facial nerve.

m. masseter

Lies on the lateral surface of the ramus of the mandible ventral to the zygomatic arch.
Projects somewhat beyond the ventral and caudal borders of the mandible.
Covered by a thick, glistening aponeurosis, and tendinous intermuscular strands are interspersed throughout its depth.
Can be divided into three layers (superficial, middle, and deep), using the change of fiber direction as a guide to the separation between the layers.

Superficial Layer

The largest part, arises from the ventral border of the rostral half of the zygomatic arch.
Its fibers pass caudoventrally and insert, partly, on the ventrolateral surface of the mandible.
Some fibers project around the ventral and caudal borders of the mandible and insert on its ventromedial surface, as well as on a tendinous raphe that passes between the masseter and the m. pterygoideus medius.
The tendinous raphe continues caudally from the angle of the jaw and attaches on the temporal bone adjacent to the tympanic bulla.
In specimens with well-developed masseters, this layer, at its ventral border, projects somewhat over the m. digastricus.

Middle Layer

The thinnest part, arises from the zygomatic arch, medial to the origin of the superficial layer and in part caudal to it.
Most of its fibers pass ventrally to be inserted on the ventral margin of the masseteric fossa and the narrow area just ventral to the fossa.
In some specimens, a small bundle of fibers, which belong to this layer, runs in a more rostral direction to be inserted on the rostroventral margin of the fossa.

Deep Layer

Impossible to isolate at its origin because many of its fibers intermingle with those of the temporalis.
Some fibers, however, arise from the medial surface of the zygomatic arch.
The majority of its fibers are directed caudoventrally and are inserted in the caudal part of the masseteric fossa and on the ridge adjacent to it. A few fibers pass ventrally along the rostral margin of the temporal muscle to be inserted on the rostral ridge of the masseteric fossa.
The masseter of pigs is a complex muscle exhibiting unique histochemical properties and sarcomeric lengths in different parts of the muscle. This contradicts earlier suggestions that carnivore masticatory muscles contain a “superfast” myosin.
Action: To raise the mandible in closing the mouth.
Innervation: N. massetericus of the n. mandibularis, from the n. trigeminus.

m. temporalis

The largest muscle of the head.
Occupies the temporal fossa, from which it extends ventrally around the coronoid process of the mandible.
During the course of its ventral extension, it is related rostrally to the orbit and orbital fat, medially to the mm. pterygoidei, and laterally to the m. masseter.
Dorsolaterally it is covered by the caudoauricular muscles, the scutiform cartilage, and the ear.
Arises largely from the parietal bone and to a lesser extent from the temporal, frontal, and occipital bones.
The margins of the muscle at its origin are the orbital ligament and temporal line rostrally, the zygomatic arch laterally, the dorsal nuchal crest caudally, and the external sagittal crest or temporal line medially. Closely applied to the muscle, within these margins, is a thick, glistening fascia. In dolichocephalic dogs the temporal muscle meets its fellow of the opposite side and forms a middorsal sulcus. In dogs with brachycephalic heads the temporal muscles usually do not meet on the midline, and the area is devoid of muscle, except for the dorsal and caudal auricular muscles.
From its large origin, the muscle fibers curve rostrally and ventrally medial to the zygomatic arch to invest and insert on the coronoid process of the mandible, as far ventral as the ventral margin of the masseteric fossa. On the lateral side of the coronoid process the fibers are intermingled with fibers of the deep layer of the m. masseter. On the medial side the fibers lie in contact with the mm. pterygoidei. A bundle of muscle fibers arise from the nuchal crest, near the base of the zygomatic process of the temporal bone, and sweeps rostrally dorsal and parallel to the zygomatic arch. It blends gradually into the main mass of the muscle.
The dog temporalis muscle contains a unique isoform of myosin common to masticatory muscles of other carnivores. This isoform has been called “superfast myosin” by others. Although the specific function of this 2M (sometimes called IIM myosin) is not known, it has been demonstrated in dog temporalis and masseter but not in appendicular skeletal muscles.
Action: To raise the mandible in closing the mouth.
Innervation: N. temporalis of the n. mandibularis from n. trigeminus.

m. pterygoideus lateralis

A much smaller and shorter muscle than the m. pterygoideus medialis.
Arises from the sphenoid bone in a small fossa, which lies ventral to the alar canal and orbital fissure.
The ventral boundary of its origin is a bony ridge also on the sphenoid bone. This short muscle passes ventrolaterally and slightly caudally, to be inserted on the medial surface of the condyle of the mandible just ventral to its articular surface
Action: To raise the mandible.
Innervation: Nn. pterygoidei of the n. mandibularis from n. trigeminus.

m. pterygoideus medialis

Arises from the lateral surface of the pterygoid, palatine, and sphenoid bones.
Passes caudolaterally to be inserted on the medial and caudal surfaces of the angular process of the mandible, and ventral to the insertion of the mm. temporalis and pterygoideus lateralis.
Many fibers insert on a fibrous raphe that passes between the insertion of this muscle and the superficial layer of the masseter muscle.
When viewed from the pharyngeal side, the medial pterygoid completely covers the lateral one.
The inferior alveolar nerve passes across the lateral face of the m. pterygoideus medialis and the medial surface of the m. pterygoideus lateralis, thus separating the two muscles.
The m. pterygoideus medialis extends to the caudal margin of the mandible and is inserted on the caudal margin and slightly on the caudomedial surface.
Action: To raise the mandible.
Innervation: Nn. pterygoidei of the n. mandibularis from n. trigeminus.

m. digastricus

Runs from the paracondylar process of the occiput to the ventral border of the mandible.
Although it appears as a single-bellied muscle in the dog, a tendinous intersection and an innervation by both the n. trigeminus and the n. facialis are evidence of its dual nature.
Thus the two parts of the muscle are referred to as the rostral belly and the caudal belly.
EMG patterns in the two bellies (rostral and caudal) in rodents suggest that activity in the caudal belly precedes electrical activity in the rostral belly. This sequential recruitment pattern allows the caudal belly to modulate the length of the rostral belly such that the latter functions at or near an optimal part of its length-tension curve.
The digastricus lies medial to the parotid and mandibular glands. After crossing the ventrocaudal edge of the insertion of the masseter, it has a fleshy ending on the ventromedial border of the mandible over a distance of approximately 2.5 cm, to the level of the canine tooth. Small muscle bundles extend far rostral toward the chin.
Action: To open the mouth.
Innervation: N. facialis to caudal belly and n. trigeminus to rostral belly.

III. Muscles of Bulbus Oculi—Extrinsic

There are seven extrinsic muscles of the eyeball: two oblique muscles, four recti muscles, and the retractor bulbi.
Closely associated with these, but inserting in the superior eyelid, is the m. levator palpebrae superioris (see Deep Muscles of the Eyes, Forehead and Ears).
All of the extrinsic ocular muscles insert in the fibrous coat of the eyeball near its equator.
The level of insertion of the recti muscles is nearer the corneoscleral junction than is that of the four parts of the retractor.
In general, the oblique muscles insert in an intermediate zone between the insertions of the recti and retractor groups.
Extraocular muscles are characterized by two layers: a surface region composed of small-diameter fibers, and a global region composed of fibers with heterogeneous diameters. Myosin heavy chain isoform expression patterns differ between the global and surface layers of dog rectus muscles with more complexity and up to nine isoforms found in the surface layer.

m. obliquus ventralis

Arises from the rostrolateral margin of a variably sized opening in the palatine bone adjacent to the suture between the palatine, lacrimal, and maxillary bones. Frequently a groove harbors the origin of the muscle and extends caudally. As the ventral oblique muscle passes ventral to the eyeball, it gradually widens and crosses ventral to the tendon of insertion of the ventral rectus. The ventral oblique divides as it reaches the ventral border of the lateral rectus. Part of its tendon crosses that of the lateral rectus superficially to attach to the sclera lateral to the insertion of the dorsal rectus. The deep part goes medial to the lateral rectus and ends in the sclera.
Action: To rotate the eyeball around its anterior to posterior axis so that the lateral part is moved laterally and ventrally—extorsion.
Innervation: N. oculomotorius.

m. obliquus dorsalis

Arises from the medial border of the optic canal. It ascends on the dorsomedial face of the periorbita to a cartilaginous pulley located on the medial wall of the orbit near the medial canthus of the eye.
The pulley, or trochlea, is a disc of hyaline cartilage located dorsocaudal to the medial canthus of the lids on the medial wall of the orbit, less than 1 cm from the orbital margin. It is spherical to oval in outline, with its long axis parallel to that of the head. It is approximately 1 cm long by 1.5 mm thick. The trochlea is suspended from the rostral border of the frontal bone and its zygomatic process by three ligamentous thickenings of the periorbita. A long, distinct ligament runs from the rostral end of the trochlea, where the dorsal oblique tendon bends around the cartilage, to the periosteum at the medial canthus. A short but wide thickening of the periorbita anchors the trochlea to the dorsal orbital wall near its margin. The third ligament is a thickening in the periorbita that runs from the caudal pole of the trochlea to the periosteum on the ventral surface of the zygomatic process. The slender tendon of the dorsal oblique muscle passes through a groove on the medial surface of the ventrorostral end of the trochlear cartilage, where it is held in place by a collagenous ligament.
As it passes through this pulley, or trochlea, it bends at an angle of approximately 45 degrees. It passes dorsolaterally and deep to the tendon of the dorsal rectus, at the lateral edge of which it inserts in the sclera. It is the longest and slenderest muscle of the eyeball.
Action: To rotate the eyeball around its anterior–posterior axis so that the dorsal part is pulled medially and ventrally—intorsion.
Innervation: N. trochlearis.

mm. recti

Include the mm. rectus lateralis, rectus medialis, rectus dorsalis, and rectus ventral