Page 369 - Anatomy and Physiology of Farm Animals, 8th Edition

354 / Anatomy and Physiology of Farm Animals

Pharynx ventral midline. In the human, the thyroid
cartilage creates the “Adam’s apple,” a pro-
VetBooks.ir The pharynx is a soft tissue conduit com- jection on the ventral aspect of the neck.

The laminae of the thyroid cartilage have
mon to both food and air caudal to the oral
and nasal cavities. The pharynx is divided processes that articulate with other carti-
into regions named for their proximity to lages and give attachment to a number of
other spaces: nasopharynx caudal to the muscles which move the larynx.
choanae, oropharynx caudal to the mouth, The cricoid cartilage is shaped like a
and laryngopharynx surrounding the lar- signet ring, with the broad portion on the
ynx (Fig. 19‐2). Foodstuffs and liquids are dorsal side. The cricoid cartilage articu-
directed into the esophagus from the lates with the thyroid cartilage and the two
mouth (and vice versa during rumination) arytenoid cartilages cranially and attaches
with a coordinated movement of pharyn- to the first cartilaginous ring of the trachea
geal and laryngeal muscles that excludes caudally.
these substances from the airways. The paired arytenoid cartilages are
irregular in shape, with processes that vary
Larynx between species. The arytenoid cartilages
of all species have a ventral vocal process,
The larynx is the gatekeeper to the to which is attached the vocal ligament
entrance of the trachea. It maintains a (“vocal cord”). The vocal ligaments mark
rigid, boxlike shape via a series of paired the division between the vestibule, the
and unpaired cartilages that are moved laryngeal space cranial to them, and the
relative to one another by striated laryn- infraglottic cavity, the space caudal to
geal muscles. The larynx’s primary func- them (Fig. 19‐5). The slitlike gap between
tion is to regulate the size of the airway and the two vocal ligaments is the rima glotti-
to protect it by closing to prevent sub- dis. Movement of the arytenoid cartilages
stances other than air from entering the changes the tension and angle of the vocal
trachea. It is capable of increasing the ligaments, closing or widening the glottis
diameter of the air passageway during (Fig. 19‐6) or adjusting the pitch of the
forced inspiration (as during heavy exer- voice as the ligaments vibrate.
cise) and closing the opening during swal- Horses and swine also possess a vestibu-
lowing or as a protective mechanism to lar (ventricular) ligament cranial to the
exclude foreign objects. Secondarily, the vocal ligament. An outpocketing of mucous
larynx is the organ of phonation (vocaliza- membrane between the two ligaments forms
tion), hence its common name, voice box. a blind pouch called the lateral ventricle.
Contraction of muscles in the larynx The vagus nerve (cranial nerve X) car-
changes the tension on ligaments that ries axons that innervate the muscles
vibrate as air is drawn past them; this vibra- (motor) and the mucous membranes (sen-
tion produces the voice. sory) of the larynx. Most of these muscles
Five mucous membrane‐lined cartilages receive their motor innervation from the
make up the larynx in most domestic ani- recurrent laryngeal nerve, which for
mals (Fig. 19‐5). The unpaired, spade‐ embryological reasons branches from the
shaped epiglottic cartilage (epiglottis), vagus within the thorax and must return
which lies just caudal to the base of the craniad along the trachea to reach the lar-
tongue, is mostly elastic cartilage. During ynx. This very long course of the axons in
deglutition, movements of the tongue and the recurrent laryngeal nerve (from the
larynx fold the epiglottis caudad so that it brainstem, where the neuronal cell bodies
covers the entrance into the larynx. reside, down the neck into the thorax, and
The thyroid cartilage resembles a taco back up the neck to the larynx) makes
shell, consisting of two parallel plates, the them susceptible to trauma and metabolic
laminae, on each side, joined on the diseases.