Page 253 - Anatomy and Physiology of Farm Animals, 8th Edition

238 / Anatomy and Physiology of Farm Animals

embedded in the rostral auricular muscles. after, for example, attending a very loud
music concert.
It functions as a fulcrum for the attached
VetBooks.ir auricular muscles. facial nerve, passes through the tympanic
The chorda tympani, a branch of the
cavity. The chorda tympani carries fibers
Middle Ear for taste from the rostral two‐thirds of the
tongue and also parasympathetic fibers
The middle ear is an air‐filled space, the destined for the mandibular and sublingual
tympanic cavity, lined by mucous mem­ salivary glands. Infections of the middle
brane and contained within the temporal ear (otitis media) can be associated with
bone. In most domestic animals, the middle dysfunction of this nerve. Likewise, sym­
ear features a ventrally expanded cavity, the pathetic fibers that innervate the eye pass
tympanic bulla, visible on the ventral sur­ through the tympanic cavity; diseases
face of the skull. The middle ear is closed to affecting the middle ear can produce signs
the external acoustic canal by the intact of lost sympathetic innervation to head
tympanic membrane and communicates structures, including a constricted pupil,
with the nasopharynx via the auditory tube sunken globe, and drooping of the upper
(formerly eustachian tube). In the horse, the eyelid (Horner’s syndrome).
auditory tube is expanded to form the large In horses, cranial nerves VII and IX
air‐filled guttural pouch dorsocaudal to through XII and sympathetic axons to
the nasopharynx. The connection between the head pass along the wall of the
the middle ear and the pharynx is normally guttural pouch, separated from its inte-
closed except briefly during swallowing. rior only by mucous membrane. Diseases
The opening of the auditory tube brought of the guttural pouch can therefore
about by swallowing or yawning permits produce distinctive neurologic dysfunc-
equalization of air pressures between middle tions when these nerves are affected.
and external ears.
Three auditory ossicles span the middle
ear from tympanic membrane to the ves­ Internal Ear
tibular (oval) window. From superficial to
deep, they are the malleus (hammer), incus The internal ear is housed entirely within the
(anvil), and stapes (stirrup) (Fig. 12‐6). petrous temporal bone. It is characterized
These tiny bones provide a mechanical by a multichambered membranous sac (the
linkage from the tympanic membrane to membranous labyrinth) closely surrounded
the vestibular window (also called the oval by a sculpted cavity of bone (the osseous
window), and through them vibrations are labyrinth). The inner ear detects both sound
transmitted from the former to the latter. and acceleration of the head (Fig. 12‐6).
The size and leverage of the auditory ossicles The membranous labyrinth in the inter­
provide mechanical advantage; the energy nal ear is a system of fluid‐filled sacs and
of pressure waves striking the tympanic ducts. The primary anatomic features of
membrane is concentrated on the smaller the membranous labyrinth are: (1) two
vestibular window, increasing the system’s enlargements, the utriculus (utricle) and
sensitivity to faint stimuli. sacculus (saccule); (2) three loops attached
There are also two striated muscles to the utriculus, the semicircular ducts;
within the middle ear, the m. tensor tym­ and (3) the spiraled cochlear duct. These
pani and the m. stapedius. These two are filled with a fluid, the endolymph. The
small muscles damp vibrations of the membranous labyrinth is housed in the
auditory ossicles in the presence of exces­ osseous labyrinth, a similarly shaped,
sively loud noises. It is persistent contraction slightly larger excavation in the petrous
of these muscles that contributes to the temporal bone. The osseous labyrinth is
temporarily reduced hearing acuity notable filled with a fluid called perilymph.