The Colon

Ignoring technical anatomical details the food canal may be described as a muscular tube about ten times the length of the body, measuring the trunk from the top of the head to the end of the spinal column. The chief part is coiled up in the lower cavity of the trunk, below the diaphragm. The upper end of the canal is controlled by the circular muscle of the lips, which is brought strongly into play in whistling. The lower end is controlled by the anus, also a voluntary circular muscle. At other points along the canal, circular muscles are placed to regulate the movements of the foodstuffs during the process of digestion. Both in health and disease these "food gates," as they may be called, have a most important relation to digestion that has not been fully appreciated until very recent times.

The upper end of the food tube is provided with a special apparatus, the mouth and teeth, for taking in food and preparing it to undergo the various processes which are carried on in the deeper parts. At the lower end of the canal is found a mechanism that is wonderfully designed to receive and discharge from the body the unused remnants of the food and other waste materials - the pelvic colon. Of this we shall learn more later.

The Structure Of The Food Tube

In structure, the food tube consists chiefly of muscle and gland tissue. There are two sets of muscle fibres. One set, the outer, runs lengthwise of the canal; an inner, circular muscle structure, surrounds the canal throughout its entire length.

Between the muscle layers is a layer of nerve cells and fibres. These are connected with the central nervous system, the brain and spinal cord, but are capable of acting independently.

The food canal is lined with mucous membrane, which presents at various points special groups of glands that produce some of the various digestive juices which act upon the food. The canal is covered through most of its course with a delicate membrane, the peritoneum.

The food tube is roughly divided into six parts - the mouth, oesophagus, stomach, small intestine, colon and rectum. The intestine is attached to the spine by a membrane, the mesentery, in which pass the nerves and blood vessels which supply the canal.

The colon may roughly be described as a muscular reservoir about five feet in length and an inch and a half to three inches in diameter. This reservoir is divided into four secondary reservoirs, the cecum, the transverse colon, the pelvic colon, and the rectum. The feces, in their preparation for discharge from the body, are passed successively from one to the other of these reservoirs, pausing for a definite interval in each, with the exception of the last.

A Normal Colon.

A Normal Colon.

Radiogram Showing Removal of First Half of Colon.

Radiogram Showing Removal of First Half of Colon.

Cecum and Appendix of a Chimpanzee.

Cecum and Appendix of a Chimpanzee.

Colon of a Japanese Deer.

Colon of a Japanese Deer.

Cecum of a Dog.

Cecum of a Dog.

Cecum of an Opossum.

Cecum of an Opossum.

Normal Position Of The Colon

When in its normal position, the colon begins at. the lower right hand section of the abdominal cavity; its head, a pouch much broader than the rest of the colon, lies in the hollow of the right iliac bone. This is the cecum. The small intestine joins the cecum about an inch and a half above its lower part, leaving a pocket, at the bottom of which is attached the appendix.

From the cecum the intestine ascends along the right side of the abdomen to the liver. This portion is the ascending colon. At the liver a rather sharp turn is made toward the left, the hepatic flexure.

From this point the colon passes across the body above the umbilicus, sloping upward toward the left side, where it lies in close contact with the spleen. This section is the transverse colon.

At the spleen the intestine makes a short turn, the splenic flexure, then passes downward along the left border of the abdominal cavity to the hip bone (crest of the ileum). This is the descending colon.

Passing obliquely across the hollow surface of the left iliac bone, the large intestine, here called the iliac colon, reaches the upper border of the pelvic cavity. Here it forms a loop, the pelvic colon, which has an average length of a foot and a half, but which varies in length from six inches to nearly three feet (in conditions of disease). The pelvic colon and iliac colon together form the sigmoid. The lower end of the pelvic colon joins the terminal portion of the intestine, the rectum, opposite the middle of the sacrum. The pelvic colon varies in position according as it is empty or filled. When empty, it falls over backward into the pelvis, and lies upon the upper part of the rectum. When it is in this position, a very pronounced fold is formed at the junction with the rectum, the pelvi-rectal fold, a factor in controlling bowel movements. When the pelvic loop is full, it rises and thus gradually obliterates the fold and so opens the passage to the rectum.

The rectum extends from the pelvi-rectal fold to the internal anal sphincter, being about six inches in length. In its upper part are two or three projecting folds of membrane known as Houston's valves.

The thicker muscular walls of the rectum are ordinarily contracted so that no cavity exists in the upper part, although some gas and often (in cases of disease) a considerable amount of fecal matter may be found in the lower part.

The distance between the internal and external sphincter is about one inch. This is the ana} canal, which is always tightly closed except during defecation.

Ileocecal Valve of a Conger Eel. A. Ileum; B. Ileocecal Valve; C. Colon.

Ileocecal Valve of a Conger Eel. A. Ileum; B. Ileocecal Valve; C. Colon.

Just above the internal anal sphincter is found a series of raised points or papillae, first described by Horner of Philadelphia many years ago. These papillae are the terminal points of special nerves which when excited cause powerful contraction of the colon and the abdominal muscles and diaphragm, and at the same time a complete relaxation of the anal sphincter.

Here are also a number of shallow pockets in the mucous membrane, the follicles of Horner, whose function is to secrete a lubricating mucus. Both follicles and papillae sometimes become inflamed and a source of pain.

Behind the rectum are located two muscles which act an important part in defecation, the levator ani muscles. In contracting, these muscles pull the anus upward and compress the rectum, and so squeeze out the last particles of fecal matter, leaving the rectum completely empty.

The small intestine is a smooth tube of uniform size, but the large intestine is sacculated. By a thickening of its muscular structures at intervals shallow pouches are formed in its sides. Along the outer surface of the colon run thick bands of muscle tissue which act in defecation like gathering strings. In contracting, these bands draw the lateral pouches together, so as to empty them of their contents. These sacs or pouches are well shown in the accompanying stereoradiogram, a rare view of the colon.

All parts of the large intestine, including the rectum, are supplied with two sets of nerves, one of which stimulates its muscles to contract, while the other exercises an opposite influence.

In the accompanying plates will be seen representations of the colons of different animals. It is especially interesting to note the close relation between the form of the colon and the character of the food in various classes of animals. In flesh-eating animals the colon is always short; in vegetable eaters it is long as compared with the body length.