Page 402 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy of the Digestive System / 387

named according to the part of the intes­ Ruminants
tine supported, that is, mesoduodenum,
VetBooks.ir mesojejunum, and mesoileum. The mes­ In the ruminant (Fig. 20‐16) the cecum is

oduodenum is generally short, so the loca­
tion of the duodenum is relatively fixed. about 12 cm in diameter, and when full of
ingesta, its blind end projects as far caudad
The mesenteries supporting the jejunum as the pelvic inlet. Cranially, the cecum is
and ileum, on the other hand, are long and continuous with the colon.
fanlike, so that many meters of intestine The proximal part of the colon is the
are connected to a small region of the ascending colon. It is modified into a series
dorsal body wall. The mesojejunum and of three loops in the ruminant. The proxi-
mesoileum are often collectively called the mal loop (ansa proximalis) forms an S
great mesentery, and the narrow stalk by shape that leads to the spiral loop (ansa
which they are attached to the body wall spiralis). The spiral colon forms an orderly
and through which blood vessels, nerves, spiraling mass embedded on the side of the
and lymphatics reach the intestines is com­ great mesentery. The first portion of the
monly called the root of the great mesen- spiral colon coils toward the center of the
tery. The length of the great mesentery mesentery (centripetally), reverses direc­
permits considerable mobility of the intes­ tion at the central flexure, then spirals away
tinal mass. from the center (centrifugally). The last
part of the ascending colon, the distal loop
(ansa distalis) connects the spiral colon
Large Intestine with the transverse colon. The transverse
colon crosses from right to left, cranial to
The large intestine consists of the cecum, a the cranial mesenteric artery, which sup­
blind sac, and the colon, which consists of plies the small intestine, the cecum, and
(from oral to aboral ends) ascending, the ascending colon, and continues caudad
transverse, and descending parts. The as the descending colon to the rectum.
descending colon terminates as the rectum
and anal canal.
There is considerably more variation in Pig
the large intestine from one species to
another than in the small intestine The porcine cecum (Fig. 20‐17) is a moder­
(Figs. 20‐15 to 20‐17). Most of this varia­ ately large (1.5 to 2.2 L) blind sac that
tion results from modifications of the projects cranioventrad near the midline.
ascending colon. The transverse colon The dorsal end of the cecum is continuous
forms a short connection that runs trans­ with the colon at the ileocecocolic junc­
versely from distal ascending colon to tion, where the entrance of the ileum marks
proximal descending colon; it is invariably the division between the cecum and colon.
found running from right to left sides of Unlike those of most domestic species, the
the abdomen, just cranial to the root of bulk of the porcine cecum lies to the left of
the great mesentery. The descending midline, with its junction with the colon
colon is generally relatively straight, run­ ventral to the left kidney.
ning caudad on the left side of the abdo­ The ascending colon of the pig, like that
men to the pelvic cavity, where it of the ruminant, presents a spiral arrange­
terminates as the rectum. An external ment of coils, although in the case of the
anal sphincter of striated (i.e., voluntary) pig the spiral loop is arranged in a cone
muscle and an internal anal sphincter of shape rather than in a flat plane. The trans­
smooth muscle characterize the walls of verse colon continues from its junction
the most distal part of the gastrointestinal with the distal end of the spiral loop, passes
tract as it opens to the exterior of the ani­ forward, then crosses to the left side of the
mal at the anus. abdomen. The bowel continues caudad as