Essentials of Anatomy and Physiology

LARGE INTESTINE

The large intestine, also called the colon, is approx-
imately 2.5 inches (6.3 cm) in diameter and 5 feet (1.5
m) in length. It extends from the ileum of the small
intestine to the anus, the terminal opening. The parts
of the colon are shown in Fig. 16–9. The cecumis the
first portion, and at its junction with the ileum is
the ileocecal valve, which is not a sphincter but serves
the same purpose. After undigested food (which is
now mostly cellulose) and water pass from the ileum
into the cecum, closure of the ileocecal valve prevents
the backflow of fecal material.
Attached to the cecum is the appendix, a small,
dead-end tube with abundant lymphatic tissue. The
appendix seems to be a vestigial organ, that is, one
whose size and function seem to be reduced. Although
there is abundant lymphatic tissue in the wall of the
appendix, the possibility that the appendix is con-
cerned with immunity is not known with certainty.
Appendicitisrefers to inflammation of the appendix,

which may occur if fecal material becomes impacted

within it. This usually necessitates an appendectomy,

the surgical removal of the appendix.

The remainder of the colon consists of the ascend-

ing, transverse, and descending colon, which encircle

the small intestine; the sigmoid colon, which turns

medially and downward; the rectum; and the anal

canal. The rectum is about 6 inches long, and the anal

canal is the last inch of the colon that surrounds the

anus. Clinically, however, the terminal end of the

colon is usually referred to as the rectum.

No digestion takes place in the colon. The only

secretion of the colonic mucosa is mucus, which lubri-

cates the passage of fecal material. The longitudinal

smooth muscle layer of the colon is in three bands

called taeniae coli. The rest of the colon is “gathered”

to fit these bands. This gives the colon a puckered

appearance; the puckers or pockets are called haustra,

which provide for more surface area within the colon.

The functions of the colon are the absorption of

water, minerals, and vitamins and the elimination of

undigestible material. About 80% of the water that

The Digestive System 385

Hepatic flexure

Haustra

Taeniae coli

Splenic flexure

Transverse colon

Ascending

colon

lleum

Descending

colon

lleocecal valve

Appendix

Cecum

Sigmoid colon

Rectum

Anus

Anal canal

Figure 16–9. The large intestine shown in
anterior view. The term flexuremeans a turn or
bend.
QUESTION:What is the function of the ileoce-
cal valve?