40 Anatomy of the Rabbit. 



THE DIGESTIVE SYSTEM. 



The digestive system comprises as its chief portions the digestive tube 

 and the digestive glands. The digestive tube is divisible into several 

 parts, which, with the exception of the caecum and its vermiform 

 process, are arranged in a linear series. The digestive glands comprise 

 the oral glands, the liver, and the pancreas. They are parts of an 

 extensive series of epithelial glands, otherwise contained within the wall 

 of the tube, and for this reason not appearing as gross structures. 



The parts of the digestive tube may be classified as follows : 



1. Oral Cavity. 5. Small Intestine. 



Oral cavity proper. Duodenum. 



Vestibulum oris. Mesenterial intestine. 



Jejunum. 



2. Pharynx. Ileum. 



Nasal portion. 



Oral portion. 6. Large Intestine. 



Laryngeal portion. Caecum. 



Vermiform process. 



3. Oesophagus. Colon. 



Rectum. 



4. Stomach. 



In its most general features the digestive system is significant as an 

 epithelial tube, in which the food during its passage is subjected to the 

 action of digestive juices provided by the epithelial glands, and is 

 modified, by solution or otherwise, so that it is capable of being absorbed 

 through the epithelial surface. In the form of the digestive tube as seen 

 in a vertebrate, however, a number of gross mechanical features are 

 evident, such as, for example, the increase in capacity, or in absorptive 

 area, through the folding of the mucous membrane, or the expansion of the 

 wall ; or again, the presence of a special muscular tunic, and its modifica- 

 tion at certain places, as in the oesophagus, the pyloric limb of the 

 stomach, and the first portion of the colon. Moreover, many features of 

 the abdominal portion of the tube, and, indeed, certain of its recognized 

 divisions, depend on its relation to an extensive serous sac — in a mammal 

 the peritoneal cavity. In this connection it is to be considered that the 

 digestive tube is primarily a median structure. It has this relation in the 

 earlier stages of embryonic development (Fig. 20), and in many of the 

 lower vertebrates it does not deviate to a great extent from a median 

 position. In all higher vertebrates, however, the tube becomes greatly 

 elongated in comparison with the cavity in which it lies, and thus becomes 

 extensively displaced to one side or other of the median plane. This 

 development, while advanced in all mammals, may be said to reach an 

 extreme in the herbivorous mammalia ; and in many cases it is further 

 increased by the independent elaboration of the blind intestine or caecum. 

 In the rabbit the combined length of the small and large intestines is 

 approximately eleven times that of the body. 



