Page 159 - Anatomy and Physiology of Farm Animals, 8th Edition

144 / Anatomy and Physiology of Farm Animals

the trigeminal nerve (see Chapter 10), the produces the range of ear movements
characteristic of domestic animals.
one exception being the caudal belly of the
VetBooks.ir m. digastricus, innervated by the facial nerve. like muscle that surrounds the palpebral
The m. orbicularis oculi is a sphincter‐
The m. temporalis is a strong muscle
arising from the sagittal crest and the fissure (the opening between the eyelids).
expansive temporal fossa of the lateral Contraction of the m. orbicularis oculi
cranium and inserting on the coronoid produces a forceful closure of the fissure.
process (a dorsally projecting blade‐like A large number of muscles produce
extension) of the mandible. Its action is to movements of the lips and cheeks; these
elevate the mandible, bringing the upper are important in prehension of food and
and lower teeth together. assist with mastication by positioning food
The m. masseter is especially well deve­ between the teeth and in the oral cavity.
loped in herbivorous species. This powerful A sphincter‐like muscle, the m. orbicularis
masticatory muscle arises from the maxil­ oris, surrounds the lips; its contraction
lary region of the face and the zygomatic purses the lips. The m. buccinator forms
arch. It inserts on the caudolateral mandible, the wall of the cheek. Other mimetic muscles
and its primary actions are to elevate the lift and depress the lips, change the shape
mandible and to draw it laterad. The broad of the nostrils, and produce other facial
expanse of the horse’s cheek is formed by movements (Table 7­4).
the masseter muscle.
Medial to the mandible are two ptery-
goid muscles (mm. pterygoidei). These Other Muscles of the Head
arise from the ventral parts of the skull (the
pterygoid and palatine bones) and insert A number of striated muscles lie within
on the mandible. These muscles assist in the orbit associated with the globe of the
closing the mandible and play an impor­ eye. These extraocular muscles (so called
tant role in the side‐to‐side grinding move­ because they are attached to the outside of
ments typical of herbivore mastication. the eyeball) move the globe of the eye in
Opening of the mouth is largely assisted the many directions of which it is capable.
by gravity, but forceful depression of the The extraocular muscles are described in
mandible is primarily the function of the Chapter 12 with the anatomy of the visual
m. digastricus. This muscle arises from apparatus.
the region caudal to the temporomandibu­ A large number of muscles associated
lar joint and inserts on the caudoventral with the pharynx and soft palate are
border of the mandible. As the name important in deglutition (swallowing)
implies, the muscle has two bellies, sepa­ and phonation (vocalization). Their vari­
rated by connective tissue near the mus­ ous functions are to lift or depress the pal­
cle’s center. ate and to constrict or dilate the pharynx.
This same region contains many muscles
Muscles of Facial Expression that attach to the hyoid apparatus. These
act to move the hyoid apparatus relative to
The muscles that move the skin and the tongue and larynx or, when the hyoid
appendages of the face and head are the apparatus is fixed in place, to move these
muscles of facial expression or mimetic latter structures relative to it.
muscles. These are generally thin cutane­ The tongue of domestic animals is an
ous muscles innervated by the facial nerve organ capable of extraordinary dexterity; it
(see Chapter 10). is used to prehend and masticate food, to
The auricular muscles are roughly drink water and suckle milk, and is used as
divided into a rostral and a caudal group. a grooming tool. The intrinsic muscles of
The contraction of auricular muscles the tongue (muscles contained entirely