Page 385 - Anatomy and Physiology of Farm Animals, 8th Edition

370 / Anatomy and Physiology of Farm Animals

• Identify and describe the main salivary continuous with one another through
Parietal and visceral peritonea are
glands.
VetBooks.ir • Describe the location and duct system of reflections of the serosa that attach the
organs to the body wall. These attachments
the pancreas.
• Distinguish between functional and collectively constitute the mesenteries,
nutritive blood supplies to the liver; and they are named for the organ they
describe the flow of these into and out of suspend (discussed later). Blood vessels,
the liver. lymphatics, and nerves travel within the
• Identify the nature and importance of mesenteries, reaching the organs through
the hepatic lobule and describe its rela­ them.
tionship to the biliary system. The wall of the digestive tract comprises
• Follow the production of bile from the four layers, or tunics. These are, from
hepatic lobule to its discharge into the within outward: (1) the tunica mucosa; (2)
intestinal lumen. the tunica submucosa; (3) the tunica
muscularis; and (4) the tunica serosa, or
(where organs lie outside body cavities)
Organization of the Digestive tunica adventitia (Fig. 20‐1).
System The tunica mucosa is the layer closest
to the space (the lumen) inside the diges­
The digestive system (digestive tract) fun­ tive tract. It has three histologic layers. The
damentally consists of a muscular tube lined innermost layer consists of a stratified
with mucous membrane that is continuous squamous epithelium from the mouth to
with the external skin at the mouth and at the level of the glandular part of the stom­
the anus. Its primary functions are prehen­ ach; from this point to the anus, the epithe­
sion, mastication, digestion (breakdown), lium is of the simple columnar type.
and absorption of food, and elimination of Underlying the epithelium of the tunica
undigested/unabsorbed wastes. Digestion mucosa are a layer of connective tissue and
reduces the nutritious constituents of the a variably present smooth muscle layer.
food to molecular compounds that are small The tunica submucosa is a layer of
enough to be absorbed and used for energy loose connective tissue in which are found
and for building other compounds for blood vessels and nerves. In some loca­
incorporation into body tissues. tions, glands of the digestive tract can be
Elements of the digestive system are the found in the submucosa, as can lymphatic
mouth, pharynx, esophagus, forestomachs nodules.
(ruminants), glandular stomach, small As motility is important to the function
intestine, large intestine, rectum, and the of the digestive system, the tunica muscu-
accessory glands (salivary glands, liver, and laris is generally well developed. In the
pancreas). horse, the cranial two‐thirds of the tunica
Caudal to the diaphragm, the compo­ muscularis of the esophagus is striated
nents of the digestive tract lie within the muscle; in the pig, all but the most distal
abdominal and pelvic cavities. Here they end of the esophagus is striated; and in
are surrounded by a simple squamous epi­ ruminants, the entire esophagus has stri­
thelium that is also called a mesothelium ated muscle. From this point and distal, the
or serosa. Within these body cavities, the muscle cells are smooth (involuntary) and
serosa is identified as peritoneum. Like the are generally arranged in two layers. The
pleura within the thoracic cavity, it is deeper layer has fibers that encircle the
named according to the structures to gut, and the more superficial muscle layer
which it is applied: where it lies directly on assumes a longitudinal arrangement.
the organ, it is called visceral peritoneum, The outermost tunic consists of the
and where it invests the abdominal wall, it visceral peritoneum and scant connective
is parietal peritoneum (see Fig. 1‐9). tissue underlying it. This is the tunica