THE DIGESTIVE SYSTEM. 



329 



meal of 200 grams of bread given to a dog, the following results were 

 obtained : 



The material obtained from the various fistulae was of course mixed 

 with the digestive juices. It is evident that the food material is 

 practically completely absorbed by the time the lower end of the ileum 

 is reached. The absorption takes place through the villi of the small 

 intestine, partly directly into the blood stream, and partly by way of 

 the lymphatic system, the food material in the latter case reaching the 

 blood-vessels along the thoracic duct. 



A villus is a finger-like projection of the mucous membrane, covered 

 by columnar epithelium, each cell having a refractive, striated border 

 on its free end, and resting by its deep extremity on a basement 

 membrane. In the centre of the villus is a lymphatic vessel, the 

 central lacteal, commencing by a blind extremity and communicating 

 with the plexus of lymphatic vessels in the submucosa. Between the 

 lacteal and the basement membrane are retiform tissue with scattered 

 leucocytes, and strands of smooth muscle which extend from the 

 muscularis mucosee and are attached to both basement membrane and 

 lacteal. A small artery is supplied to each villus and breaks up into 

 a plexus of capillaries, lying immediately under the basement membrane 

 and reuniting to form a small vein. 



THE PROCESS OF ABSORPTION. 



The absorption of the food products is effected by what, in the 

 absence of a more precise definition, is called the vital activity of the 

 epithelial cells of the intestinal mucous membrane. Experiments show 

 that the process cannot be accounted for by filtration, diffusion, and 

 osmosis. Filtration cannot take place, because the pressure in the blood 

 capillaries of the mucous membrane is higher than the pressure in the 

 lumen of the intestine. Again, a solution which is isotonic with the 

 blood serum undergoes absorption, and an animal will x even absorb its 



