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: (_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 Lieberkuehn, small caecal 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 Lieberkuehn. 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.