3-10. THE LARGE INTESTINE
The large intestine is about 5 feet long and extends from the end of the ileum to
the anus. It is divided into the cecum, colon, rectum, and anal canal (figure 3-1). At the
beginning of the large intestine, the food is moved slowly and some undigested
materials are digested (by enzymes carried over from the small intestine) and absorbed.
The main function of the large intestine is to absorb water from the residue.
a. The cecum, the commencement of the large intestine, is a large, blind pouch
in the right iliac fossa. A worm-shaped tube, the appendix or vermiform process, is
attached to the lower end of the cecum on the posteromedial aspect. The distal end of
the ileum communicates with the cecum by way of the ileocecal valve.
b. The colon, although one continuous tube, is subdivided into ascending,
transverse, descending, and sigmoid parts.
(1) The ascending colon begins at the cecum opposite the ileocecal valve
and passes upward to the lower surface of the right lobe of the liver, where it bends
abruptly forward and to the left forming the right colic, or hepatic, flexure.
(2) Beginning at the hepatic flexure, the transverse colon passes with a
downward convexity across the abdomen into the left hypochondriac region of the
abdomen, where it bends abruptly on itself beneath the lower end of the spleen to form
the splenic flexure.
(3) From its beginning at the splenic flexure (left colic), the descending
colon passes down the left side of the abdomen to its junction with the sigmoid colon.
(4) The sigmoid colon begins at the terminus of the descending colon. It
forms an S-shaped loop and normally lies within the pelvis. It passes across the front of
the sacrum to the right side of the pelvis; it then curves on itself and follows a leftward
course to the midline at the level of the third sacral segment, where it bends downward
and ends in the rectum.
(5) The rectum extends downward to the anal canal. It is continuous with
the sigmoid colon and anal canal. The rectum is about 4-1/2 inches (12 cm) long and,
at its distal end, it is dilated to form the rectal ampulla.
c. The anal canal is the terminal portion of the large intestine and measures
from 2.5 to 4 centimeters in length. It extends from the distal end of the rectum and
terminates in the external opening of the alimentary tract, the anus (figure 3-1).