278 ESSENTIALS OF ZOOLOGY 



crystalloid state. Absorption takes place through the villose 

 walls, most of the products passing into the blood, but the 

 fats into the lymphatics, which, from the milky contents after 

 a meal, have been called lacteals. 



The undigested residue is passed into the large intestine. 

 The place of entrance of the small intestine is guarded by a 

 valve, the ileo-caecal valve, and at its termination the ileum 

 bears a large accumulation of lymphoid tissue, the sacculus 

 rotundus. The large intestine divides at once into two 

 branches. A blind branch, the caecum, is a long, wide tube 

 spirally constricted and ending in a narrow tube containing 

 a large amount of lymphoid tissue. This is the vermiform 

 appendix, and has the same composition as the sacculus 

 rotundus and Peyer's patches. The other branch of the large 

 intestine is the colon. It is also a wide tube with constricted 

 walls, but it gradually loses the folds and becomes smooth- 

 walled, passing into the rectum, which leads straight back 

 to the anus. It presents the round masses of undigestible 

 residue which are passing to the exterior as egesta or faeces. 



The most important substance left over from the digestive 

 activities of the small intestine is cellulose, and it is attacked 

 in the large, intestine and especially in the caecum. Time is 

 required for the process, and this is secured in the blind pouch 

 of the intestine, always well developed in animals which 

 feed to a large extent on vegetable food. The process is 

 aided by the heat of the body, the awakening of enzymes 

 present in the food, and to a large extent evidently by the 

 aid of bacteria. The products, together with water and other 

 soluble substances, are absorbed through the mucous mem- 

 brane of the caecum and colon. 



With the exception of the fats, the food, after absorption 

 through the thin mucous membrane of the intestine, is carried 

 to the liver, and it undergoes in passing through the capillaries 

 of the liver some degree of analysis. The carbohydrates 

 are stored in the liver in the insoluble state of glycogen, and 

 only paid out into the blood leaving the liver as required ; toxic 

 substances are neutralised. In short, the blood leaving the 

 liver is different from that entering by the portal vein, in so 

 far as food is concerned. In the blood, food is distributed all 



