The large intestine constitutes the egressive portion of the digestive tract through which the residue of the food mass is forwarded for expulsion after the usable part has been diverted through the walls of the small intestine into the circulating blood. Its diminished importance in the essential work of the digestive tract may account in part for the relative absence of structural modifications designed to increase its inner surface.
Except in the embryo it is without the villi which characterize the small intestine, and although Lieberkiihn glands are present in lessening abundance along its lining, whatever glandular secretions are present are not so much concerned with chemical digestion as with mechanical lubrication of the feces from which the water content has been largely withdrawn.
The large intestine in man, which is about five feet in length, is differentiated into the colon, with ascending, transverse (from right to left), and descending portions, and finally the rectum, ending at the anal opening. The colic caecum and the processus vermiformis, already mentioned, properly belong to the large intestine, although they occur at the junction between the small and the large intestine. The same parts characterize the large intestine of many mammals but are not so distinctive in other vertebrates, since only the rectum of mammals is homologous with the large intestine of lower vertebrates. In the lower fishes as well as in the pipefish, Syngnathus, the stickleback, Gasterosteus, and others, the “large intestine” is actually smaller than the small intestine.
In man the colon, which is looped around the small intestine, is characterized by three bands of longitudinal muscles, the teniae coli, that pull this part of the intestine together so as to form three rows of pouches, or haustra, along its entire length, a modification present in varying degree in the large intestine of other mammals. Attached to the external wall of the haustra there are also numerous small processes of connective tissue, often distended with fat, called the glandulae epiploicae, but neither the haustra nor the glandulae epiploicae extend to the rectum.
In birds the rectal region of the large intestine is notably reduced. Since it is a disadvantage for these aerial creatures to carry about an unnecessary weight of useless fecal material, the provision for its temporary retention in a large intestine is curtailed, the excreta being disposed of as rapidly as assembled.
In many vertebrates, including monotremes among mammals, the urinary and genital ducts enter the posterior part of the large intestine, which thus receives not only the feces from the digestive tube but also the products of the excretory and reproductive organs. This region, the common exit for all three systems, is known as the cloaca. In most mammals the cloaca splits longitudinally during embryonic development, thereby separating the more ventral urinary and genital pathways from the more dorsal rectal portion concerned solely with the feces.
The rectum ends with the anus, which is kept closed by a sphincter muscle under the control of the will, unlike other circular muscles of the digestive tract that accomplish segmentation and peristalsis under involuntary control. The inner walls of the rectum in man are modified by two or three transverse crescentic shelflike folds, the anal valves, which are doubtless adaptations to erect posture, since they are absent in quadrupeds where the feces do not weigh against the anal sphincter.
Metchnikof, as well as others, has pointed out that the large intestine in man, particularly in the rectal region, is a danger zone, a veritable “sewage swamp,” because bacteria of various kinds flourish upon the undigested residue of the food retained there, contaminating the body constantly with the fermentive and toxic by-products of their metabolism. In one tabulation of 1148 cases of cancer of the alimentary tract, eighty-nine per cent were located in the susceptible region of the rectum.
There are some obvious advantages to mankind in possessing a voluminous large intestine and rectum where excreta may be held temporarily so that their evacuation may be timed and controlled more readily than in the case of flying birds, although the disadvantages are far from negligible. It is somewhat difficult to see how so unfortunate an evolutionary acquisition could have been fostered by natural selection, but the fact that it reaches its greatest elaboration in herbivorous animals, where it is still useful in caring for plant foods rich in cellulose, may give some hint of why such a danger zone came to be acquired and preserved in man.