In the warm-blooded animals (mammals and birds) the tissue-changes are constant and rapid and call for a large amount of nutrition within a given period, while the metabolic processes in the cold-blooded vertebrates (reptiles, amphibia and fishes) are slow, these animals being able to go without food for long periods. Consequently in the former class the small intestine is relatively much longer than in the latter. Thus in certain birds and herbivorous mammals the small intestine exceeds the total length of the body many times. This influence of the quantity and quality of the food on the length of the intestinal canal is seen, for example, very well during the course of development in the frog.
The increase in the length of the intestine, and the consequent varying degrees of coiling and convolution, are therefore secondary acquired characters, depending for their development upon the habitual kind and volume of the food. Additional provisions for increasing the efficiency of the digestive apparatus are encountered throughout the whole of the intestinal canal. In many forms the digestive secretory and absorbing area is augmented by the development of folds, valves, diverticula, villi and papillæ from the mucous surface of the intestine. Certain valves and folds, moreover, both control the direction in which the contents of the canal move and retain the same for a longer period in the intestinal segment in which they develop. Such folds appear especially well developed in the intestine of certain cyclostomes, selachians and dipnœans (cf. [Figs. 203] and [204]). In these forms the alimentary canal is usually short and straight, and the fold which has a typical spiral course and projects far into the lumen of the gut, evidently makes up to a very large extent for the shortness of the intestine, serving the threefold purpose of
(a) Increasing the digestive and absorbing surface;
(b) Prolonging the period of retention of the food-substances in the intestine, and thus increasing the time available for elaboration and absorption.
(c) Regulating the direction in which the intestinal contents move.
We will see presently that a similar spiral mucous fold is also encountered in some of the higher vertebrates, especially in the large intestine. Examples are found in the well-developed spiral valve in the cæca of the ostrich ([Fig. 341]), the similar fold in the large intestine of many rodents ([Figs. 387] and [388]) and in the crescentic plicæ of the primate large intestine ([Figs. 471], [472] and [473]).
To the same physiological category belong the digestive diverticula of the intestinal canal, such as the pyloric appendices of the midgut found in many teleosts and ganoids (cf. [p. 119]) and the varieties of cæca or blind diverticula of the hindgut encountered throughout the vertebrate series. They all function as reservoirs which increase the available digestive and absorbing surface and which in addition are especially adapted to retain substances difficult of digestion until the processes of elaboration have been completed.
Divisions of the Small Intestine.—In the higher forms the segment of the small intestine which succeeds to the pylorus is distinguished as the duodenum. Into it empty the ducts of the liver and pancreas. In some animals a pear-shaped enlargement is found, corresponding to the duodenal antrum of the human intestine, as the dilated proximal portion of the duodenum immediately beyond the pylorus is called. Examples of this condition are furnished by the cetaceans, several rodents, the llama and dromedary and the koala (Phascolarctos).
In the birds and in many mammals (e. g., dog, [Fig. 200], and many rodents, as the rabbit) the duodenum is drawn out into a long loop surrounding the pancreas.
Structure of the Small Intestine. 1. Secretory Apparatus.—The glands whose ducts empty into the small intestine and which furnish the digestive secretions, may be divided as follows: