ALIMENTATION 119 



circulation. Since liver cells show an accumulation of fat during 

 absorption, it follows that some of the fat is taken out of the 

 blood by the liver (Fig. 73). Chyle is removed from the lacteals 

 by contractions of the musculature of the villi. 



Fat is used by the tissues for heat production, the final products 

 of oxidation being carbon dioxide and water. Fats like carbo- 

 hydrates are energy producers. An excess may be stored in the 

 body as adipose tissue, a reserve which may be drawn upon 

 when needed. 



Digestion and Absorption in the Large Intestine. — The 

 mammalian large intestine does not produce specific digestive 

 enzymes. The same is also true of the frog. The secretion of 

 the human large intestine consists of mucus and is alkaline in 

 reaction. However, since the contents received by the large 

 intestine from the small intestine contain some undigested food 

 together with enzymes and since the large intestine retains its 

 contents for a long time, digestion and absorption continue 

 there as in the small intestine. The outstanding function of the 

 large intestine is the absorption of water as a result of which 

 the contents are dried to the consistency of the feces. 



Bacterial Digestion. — The intestine of man as well as that of 

 other animals normally contains large numbers of bacteria. In 

 the small intestine they cause fermentation of carbohydrates, 

 which results in the production of carbon dioxide, water, alcohol, 

 and acetic acid. These substances are probably absorbed and 

 used by the body, but they could be used equally well if absorbed 

 as unfermented sugar. In the large intestine the same thing 

 may take place, but usually bacterial action in this part of the 

 digestive tract is on undigested fragments of protein, causing 

 putrefaction. Autointoxication results from the absorption in 

 the large intestine of toxic substances formed in the process of 

 putrefaction. 



