Myology of the Head

• This document covers the myology of the head, focusing on muscles of the lips, cheeks, eyelids, mandible, hyoid apparatus, tongue, and pharynx.
• The cranial nerves involved in innervation are: Facial nerve (CN VII), N. mandibularis, N. trigeminus (CN V), N. hypoglossus (CN XII), N. glossopharyngeus (CN IX) & N. vagus (CN X)
• The muscles are described by their general characteristics, origin, insertion, function, and innervation.

Muscles of Lips, Cheeks, and Eyelids

• M. orbicularis oris is a sphincter-like muscle that surrounds and closes the lips and is innervated by the facial nerve (CN VII).
• M. orbicularis oculi is a sphincter-like muscle that surrounds and closes the eyelid opening.

Mandibular Muscles

• These muscles lie lateral, medial, and caudal to the mandible, with species differences due to diet.
• M. masseter has two layers separated by a fibrous plate that is well-developed in herbivores.
• Origin of M. masseter is at the Arcus Zygomaticus, Crista facialis (horses)/Tuber faciale (bovine), inserting into the Fossa masseterica of the mandible or lateral surface of Proc. coronoideus (dog).
• Bilateral contraction causes closure of the mouth, while unilateral contraction moves the mandible laterally. Nerve innervation is N. mandibularis, N. trigeminus (CN V).
• M. temporalis occupies the Fossa temporalis. In horses, the ventral part is partially fused to M. masseter. In dogs, it's the largest and strongest muscle of the head. In bovine, it is poorly developed and situated laterally.
• The origin of M. temporalis is the Fossa temporalis, inserting into the Proc. coronoideus, and assists in closure and retraction of the mandible. Its innervation is primarily the N. mandibularis (CN V).

Muscles of the Hyoid Apparatus

• The hyoid apparatus suspends and supports the root of the tongue and larynx and is involved in swallowing.
• M. sternohyoideus runs ventrally to the trachea, originating at the Manibrium sterni and inserting into the Basihyoideum.
• M. sternohyoideus retracts the hyoid apparatus and tongue at the end of swallowing, and is innervated by ventral branches of cervical spinal nerves and N. accessories (CN XI).
• M. omohyoideus is not in dogs, but in horses is a thin, strap-like muscle with a fascia originating deep to cranial border of the scapula. In Bovine it is cranially at C3 & C4 and inserts to the Basihyoideum.
• M. omohyoideus retracts the hyoid apparatus and tongue and is innervated by ventral branches of cervical spinal nerves.
• M. thyrohyoideus extends from the thyroid cartilage to the Thyrohyoideum, draws hyoid apparatus caudally and/or larynx rostrally and is innervated by N. hypoglossus (CN XII).
• M. mylohyoideus is thin. Left and right muscles meet and fuse in midline forming fibrous raphe. Forms Intermandibular sling for tongue, arising from Linea mylohyoidea on lingual surface of Corpus mandibulae inserting into the Basihyoideum.
• M. mylohyoideus lifts body and protects the hyoid apparatus and is supplied by N. mylohyoideus of N. mandibularis (CN V).
• M. geniohyoideus are two long fusiform muscles which lie next to either side of the median plane extending from Pars incisive of mandible inserting into the Basihyoideum, resulting in the protraction of the tongue and hyoid apparatus an dis innervated by N. hypoglossus (CN XII).
• M. stylohyoideus is a slender, strap-like muscle from Angulus stylohyoideus Dog=tympanohyoid end of stylohyhoideum inserting into the Thyrohyoideum/Basihyoideum. In horses, the tendon of insertion is perforated by intermediate tendon of M. digastricus.
• M. stylohyoideus elevates and retracts the larynx and root of the tongue, and during swallowing, elevates the Basihyoideum, innervated by N. facialis (CN VII).

Muscles of the Tongue

• M. lingualis proprius is an intrinsic muscle comprising a symmetrical arrangement of muscle fibres from the Basihyoideum and results in licking, lapping, food prehension, mastication, and swallowing and is innervated by N. hypoglossus (CN XII).
• M. genioglossus is a muscle from the Pars incisive inserting into the Basihyoideum Ceratohyoideum drawing the tongue rostrally and also innervated by the N. hypoglossus (CN XII).
• M. hyoglossus is an extrinsic muscle originating from Lateral basihyoideum /thyrohyoideum inserting into the Caudal part of tongue retracting and depressing the tongue.
• M. styloglossus is an extrinsic muscle where the dorsal end of Stylohyoideum inserts into the Ventro-lateral & ventral surfaces of tongue which retracts the tongue.

Muscles of Eyelids

• M. levator palpebrae superioris lies between the Periorbita and extrinsic eye muscles, the tendon of which blends with M. orbicularis oculi, and its action is retraction of the upper eyelid.

Extrinsic Muscles of the Eye

• Mm. recti describes four straight muscles are located around the optic canal and insert into of the Corneal margin of eyeball.

Muscles of the Pharynx

• These muscles are straited, respond to the swallowing reflex, are under voluntary control, and are innervated by the N. glossopharyngeus (CN IX) & N. vagus (CN X),
• M. stylopharyngeus caudalis is a pharyngeal dilator which originates from the proximal stylohyoid bone and inserts dorsally to the pharynx.

Laryngeal Cartilages

• The larynx is a tubular, cartilaginous structure linking the pharynx and trachea, and is suspended by the hyoid apparatus, and prevents inhalation of foreign material
• The epiglottis is rostral with a triangular shape with a base attached to thyroid cartilage (Petiolus epiglottidis-stalk) and closes off the laryngeal opening when swallowing,.
• Within horses the stalk has Proc. Cuneiformis on either side. The Faces lingualis is convex and the Faces laryngeais is concave
• The thyroid is the main central component with 2x vertical laminae that unite ventral except in the horse (Caudal margin - Incisura thyroidea). It is also U-shaped.
• Each laminae of the thyroid has a Cornu rostrale and caudale (Rostral horn articulates with Thyrohyoideum and caudal with cricoid cartilage)
• Notches between horns on the thyroid = Incisura thyroidea rostralis and caudalis
• The Cricoid is shaped like a singet ring with a broad dorsal lamina (Lamina cartilaginis cricoideae) and narrow arch (Arcus cartilaginis cricoideae).
• The cricoid has an Articular facet for Proc. Mascularis (Arytenoid cartilages) , is attached caudally to tracheal ring and articulates to caudal horn of thyroid cartilages
• Paired arytenoid are paired at the entrance of the larynx and are located caudal to the epiglottis and rostral to cricoid. The are the smallest cartilage and has many processes: Proc. Muscularis articulates to cricoid and Proc. Vocalis
• Proc. corniculatus is located dorsally (Horse) and on the dog equals the Proc. Cuneiformis

Muscles of the Larynx

• The extrinsic muscles connect the larynx to the sternum and basihyoid bone, moving the larynx as a unit.
• The intrinsic muscles connect different cartilages and move them relative to each other and are ennervated by the branches of CN X, N. vagus
• M. sternothyroideus is an extrinsic muscle on the ventro-lateral side on the trachea that retracts the larynx, hyoid apparatus and tongue
• M. cricoarytenoideus dorsalis originates from the Cricoid lamina inserting into the Proc. aretynoid muscular process opening the Rima glottidis laryngeal entrance and is innervated by the N. laryngeus recurrens

Mechanical Process of Mastication

Dog mastication:

• M. diastricus is well developed which opens the jaw wide via a wide zygomatic arch + Absence of osseous caudal rim creating room for M. Temporalis without limiting gape
• Gaps are limited by Masseter and medial pterygoideus with mandible leverage, consisting of Size of Porc, coronoideus muscle insertion and Ventral pots on go articular condole that distributes load oh cadnace .
• Carnassial teeth (Maxilla → 4th premolar and Mandible → 1st molar) Closer to axis of movement

Herbivores mastication involves a repeated Cycles of rotary grinding movements of the mandible and results in poorly developed Temporal is and with the mail muscles from the Maseru and the pterygoideus.

• Mechanical advantage of Articular surface of cheek is a distance from occlusal surface of cheek teeth

Extrinsic Muscles of the Ear

• The abductors, adductors, levators, depressors and rotators re highly developed and have a wide range of movement which activate the external car Auricula and are unnervsted by Civ vii