Parts of the Large Intestine

The large intestine consists of the cecum, the colon, the rectum, and the anal canal. Figures 17.45 and 17.46 and reference plates 51, 52, 58, and 75 depict the large intestine.

The cecum, at the beginning of the large intestine, is a dilated, pouchlike structure that hangs slightly inferior to the ileocecal opening. Projecting downward from it is a narrow tube with a closed end called the vermiform (wormlike) appendix. The human appendix has no known digestive function. However, it contains lymphatic tissue.

In appendicitis, the appendix becomes inflamed and infected. Surgery is required to prevent the appendix from rupturing. If it breaks open, the contents of the large intestine may enter the abdominal cavity and cause a serous infection of the peritoneum called peritonitis.

The colon is divided into four portions—the ascending, transverse, descending, and sigmoid colons. The ascending colon begins at the cecum and extends upward against the posterior abdominal wall to a point just inferior to the liver. There it turns sharply to the left (as the right colic, or hepatic, flexure) and becomes the transverse colon. The transverse colon is the longest and most movable part of the large intestine. It is suspended by a fold of peritoneum and sags in the middle below the stomach. As the transverse colon approaches the spleen, it turns abruptly downward (as the left colic, or splenic, flexure) and becomes the descending colon. At the brim of the pelvis, the descending colon makes an S-shaped

Muscular layer

Serous layer Hepatic flexure

Mucous membrane Transverse colon

Ascending colon

Ileocecal sphincter Orifice of appendix

Muscular layer

Serous layer Hepatic flexure

Mucous membrane Transverse colon

Ascending colon

Ileocecal sphincter Orifice of appendix

Hepatic Flexure Bleeding

Splenic flexure

Tenia coli

Descending colon

Haustra

Cecum Vermiform appendix

Sigmoid colon

Anal canal

Splenic flexure

Tenia coli

Descending colon

Haustra

Cecum Vermiform appendix

Sigmoid colon

Anal canal

Figure 17.45

Parts of the large intestine (anterior view).

curve, called the sigmoid colon, and then becomes the rectum.

The rectum lies next to the sacrum and generally follows its curvature. It is firmly attached to the sacrum by the peritoneum, and it ends about 5 centimeters inferior to the tip of the coccyx, where it becomes the anal canal (fig. 17.47).

The anal canal is formed by the last 2.5 to 4.0 centimeters of the large intestine. The mucous membrane in the canal is folded into a series of six to eight longitudinal anal columns. At its distal end, the canal opens to the outside as the anus. Two sphincter muscles guard the anus—an internal anal sphincter muscle, composed of smooth muscle under involuntary control, and an external anal sphincter muscle, composed of skeletal muscle under voluntary control.

Hemorrhoids are, literally, a pain in the rear. Enlarged and inflamed branches of the rectal vein in the anal columns cause intense itching, sharp pain, and sometimes bright red bleeding. The hemorrhoids may be internal (which do not produce symptoms) or bulge out of the anus. Causes of hemorrhoids include anything that puts prolonged pressure on the delicate rectal tissue, including obesity, pregnancy, constipation, diarrhea, and liver disease.

Eating more fiber-rich foods and drinking lots of water can usually prevent or cure hemorrhoids. Warm soaks in the tub, cold packs, and careful wiping of painful areas also helps, as do external creams and ointments. Surgery—with a scalpel or a laser—can remove severe hemorrhoids.

Bloody Scalpel

Figure 17.46

Radiograph of the large intestine.

Figure 17.46

Radiograph of the large intestine.

External anal sphincter

External anal sphincter

Large Intestine Anal Canal

Anal columns

Internal anal sphincter

Figure 17.47

The rectum and the anal canal are located at the distal end of the alimentary canal.

Anal columns

Internal anal sphincter

U What is the general function of the large intestine? ^9 Describe the parts of the large intestine.

^9 Distinguish between the internal sphincter muscle and the external sphincter muscle of the anus.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • quinn
    What extends from the hepatic flexure to the splenic flexure?
    5 years ago
  • haben sayid
    What blood vessels enter the large intsestine?
    4 years ago

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