Page 386 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Digestive System / 371

Serosa
Longitudinal smooth muscle
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Myenteric plexus
Submucosal
gland Circular
smooth muscle
Submucosa
Villi
Muscularis
mucosae
Epithelial cells
Mucosa

Mesentery

Lumen
Mucosa
Submucosa
Circular smooth muscle
Longitudinal smooth muscle
Serosa
Muscularis mucosae
Figure 20-1. Layers of a typical segment of gut.

serosa. The thoracic and cervical parts of The entrance into the mouth is defined
the esophagus are not suspended directly by the lips, the appearance and mobility
within a body cavity (the thoracic esopha­ of which vary among species. The exter­
gus is surrounded by the tissues of the nal parts of the lips are covered by typical
mediastinum) and therefore do not have a haired skin, which changes to mucous
tunica serosa. Instead, the surrounding membrane at the mucocutaneous junc-
connective tissue in these locations consti­ tion of the margins. The upper lip of
tutes a tunica adventitia. Likewise, the small ruminants is deeply grooved with a
distal end of the rectum and anal canal are midline philtrum. Lips are densely inner­
outside the pelvic cavity and surrounded vated by sensory fibers, making them
by a tunica adventitia. very sensitive tactile organs. The lips of
sheep, goats, and horses are soft and flex­
ible and aid in picking up food, whereas
Mouth those of cattle and hogs are stiffer and
less mobile.
The mouth is used primarily for acquiring, The small space between the teeth and
cutting and/or crushing, and mixing food lips/cheek is the oral vestibule. The oral
with saliva but may also be used to manip­ cavity proper lies deep to the teeth and is
ulate the environment (through grasping occupied primarily by the tongue. The oral
of objects) and as a defensive and offensive cavity ends at a narrowing created by folds
weapon. of mucous membrane near the base of the