There are principally three kinds of peris taltic movements of the digestive tract, one, the regular peristalsis just described and shown to be of a twofold character by Bayliss and Starling; secondly, the rhythmical movement; and third, the anti-peristaltic movement. We will now describe the rhythmical movements. Whilst the food progresses onward during the regular peristaltic movement just described, it seems to remain in the same place during the rhythmical movement but is apparently re peatedly kneaded and what is at one time a string of food of the shape of a sausage, continuous in one piece six or eight inches long, may be suddenly segmented into five or six small pieces. These may be recombined, only to be separated again. During these rhythmical contractions there is no steady progression of the food, although it is repeatedly subjected to divisions. From time to time, however, the sep arated pieces are caught by an advancing peris taltic wave of the first kind, moved forward about 6 to 10 inches and then gathered into a new mass which, in turn, undergoes segmenta tion. F. P. Mall has suggested that these rhythmical contractions may also act as a pumping mechanism upon the venous plexuses in the walls of the intestine and thus aid in driving the blood into the portal system. Sim ilar movements have been described by Hertz in the human being.
Anti-peristalsis occurs under normal condi tions nowhere except in the colon, particularly in the ascending colon. Under abnormal con ditions, for instance, during complete intestinal obstruction, it may occur anywhere in the bowels.
The time required for the passage of the food from the mouth to the stomach is six seconds, if solid; one to two seconds, if liquid. The passage through the stomach may require from two to four hours, according to the quan tity and quality of the food. The passage through the smaller intestine, according to Hertz, is about four hours and a half, though Hemmeter has described in the human being, studied by means of the X-ray, the passage of a meal from the pylorus to the ileo-ecal valve in two hours. The colon has the slowest peristal tic movement. The passage of the food through the colon may require from 10 to 14 hours.
Comparative Physiology of Digestion.— Digestion occurs . in unicellular animals, such as the protozoa, in which Hemmeter has de scribed the secretion of an acid into the food vacuole. When food that is stained with acid indicators, for instance, Congo red, is taken into the food vacuole of an ameba, it changes to blue, showing that an acid has acted upon the food.
The next highest class of animals, the Cce lenterates, possess a mouth and an alimentary tube which opens into the body cavity. A cer
tain chemical digestion takes place in this tube and the digested food is absorbed through the cells of the endoderm, the lining membrane of this digestive tube. In the Medusa fine canals radiate from the body cavity into the digestive tube and form part of the so-called gastro vas cular system. In the Echinodermata we have a further development, a complete alimentary canal with mouth and anus, and entirely shut off from the body cavity. In many Arthropods it is possible already to distinguish parts corre sponding to the stomach and the small and large intestines of higher forms, the digestive glands being represented by organs which in some groups seem to be homologous with the liver, and in others with the salivary glands of the higher vertebrates. A few Molluscs seem in addition to possess a pancreas.
Among Vertebrates, fishes have the simplest and birds and mammals the most complicated alimentary system. The stomach in the lower fishes is only indicated by a slight widening of the anterior part of the digestive tube. There are no salivary glands in water-living Verte brates. The cesophagus is generally dilated to form a crop, in Birds, from which the food passes into a stomach consisting of two parts, one pre-eminently glandular (proventriculus), the other pre-eminently muscular (ventriculus). Among Mammals a twofold division of the stomach is distinctly indicated in rodents and cetacea, but this organ reaches its greatest com plexity in ruminants, which possess no fewer than four gastric pouches. The differentiation of the intestine into small and large intestines and rectum is more -distinct, both anatomically and functionally, in Mammals than in lower forms; but there are marked differences be tween the various mammalian groups, both in the relative size of the several parts of the digestive tract and the proportion between the total length of the alimentary canal and the length of the body. In general, the canal is longest in herbivore, shorter in carnivora. Thus the ratio between length of body and length of intestine is in the cat 1:4, dog 1:6, man 1 :5 or 6, horse 1:12, cow 1:20, sheep 1:27. The relative capacity of the stomach, small intestine and large intestine is in the dog 6:2:1'5, in the horse 1:3'5:7, in the cow 7:2:1. The area of the mucous surface of the alimentary canal is very considerable, in the dog more than half that of the skin, the surface of the small intes tine being three times that of the stomach and four times that of the large intestine. In the horse the mucous surface has twice the area of the skin.