(a) Glands situated in the substance of the intestinal walls.
Two kinds are distinguished:
1. Brunner’s glands, small acinous glands confined to the first part of the duodenum.
2. Glands of Lieberkühn, small cæcal pits distributed not only over the entire small intestine, but also found in the mucous membrane of the large intestine.
These structures furnish the intestinal juice, whose chief function is the conversion of starches into sugar, while aiding in carnivorous animals also the digestion of proteid substances. The glands are hence best developed in herbivora, while in carnivora they are present in diminished numbers since they assist in the digestion of proteid substances.
The size and number of these glands also depends on the amount of food digested within a given period. When a considerable quantity of digestive fluid is required, in order to obtain the nutritive value of the food for the organism rapidly, the glandular apparatus of the intestine will be well developed. Hence mammalia, in whom these conditions exist, possess both the glands of Brunner and of Lieberkühn. In birds the latter structures are still found, but the former are absent, while amphibia and fishes are devoid of both kinds. In these lower vertebrates the typical intestinal glandular apparatus of the higher forms is to a certain extent replaced by small pits and depressions of the mucous membrane bounded by reticular folds.
(b) Glands situated outside the intestinal tube, into whose lumen their ducts empty.
The liver and pancreas fall under this head. The liver functions in the digestion of the fatty substances of the food, while the secretion of the pancreas converts the starches into sugars, and aids in the digestion of albumenoid substances and to a lesser degree in that of the fats.
2. Absorbing Apparatus of Small Intestine.—The mucous membrane of the intestine is provided with villi, containing lymphatics, by whose agency the digested matters are absorbed. These structures are developed in individual forms in direct proportion to the ease and rapidity with which the food is habitually absorbed.
The more rapid and complete the digestion is the greater will be the amount of digested nutritive material at any given time in the intestine, and the greater will be the development of the absorbing structures. Hence the villi of the small intestine are especially large and prominent in the carnivora, while they are small and insignificant in herbivora and omnivora. Intestinal villi are found in nearly all mammals and in many birds. Fig. 300 shows the villi of the intestinal mucous membrane in a carnivore mammal (Ursus maritimus, polar bear) and Fig. 301 the same structures in the cassowary (Casuarius casuarius) in which bird they are very well developed. The villi are not confined to the two highest vertebrate classes, but are encountered also in the mucous membrane of the midgut in certain reptiles, notably the ophidia.