MAN 301 



becomes acid again. The liquefiable nutriment is gradually 

 extracted from the food and the residue of undigested 

 material is finally eliminated through the anus. In a 

 healthy man digestion is usually influenced by numerous 

 bacteria which live in the alimentary canal. Some in- 

 vestigators assert that these unsuspected guests are 

 absolutely indispensable for proper digestion. 



The absorption of food is selective not simply a soaking 

 of liquids through the lining of the alimentary canal. .If 

 fresh blood is placed in the intestine it is not transferred 

 unchanged to the adjacent blood-vessels but goes the usual 

 long round of digestion, absorption, and assimilation. The 

 proteins are mostly changed to amino acids and usually 

 pass through the lining of the intestine into the blood-ves- 

 sels; the fats are emulsified, broken up into fatty acids 

 and glycerin and taken up by the lac teals;* the carbo- 

 hydrates for the most part pass as monosaccharids in 

 the blood to the liver, where they are stored as glycogen 

 or transferred to the tissues to be burned for energy 

 production. 



Most of the activities concerned with digestion are 

 under the control of the nervous system, but usually take 

 place without the knowledge of the person who has eaten. 

 Vomiting is a safeguard for rejecting poisonous or unde- 

 sirable substances; headache may be an indication of an 

 irritated stomach. Yet many of the digestive and assimi- 

 lative processes are controlled by specific chemical sub- 

 stances, which may not even enter the alimentary canal. 

 The pancreas not only manufactures a powerful digestive 

 secretion which it pours into the intestine, but also acts 

 as a ductless gland which is of great importance in con- 

 nection with the utilization of sugar from the blood. When 

 it is diseased the metabolism of carbohydrates is accord- 

 ingly disturbed. Other ductless glands (thyroid, thymus, 

 adrenals, gonads, etc.) all have more or less influence on 



* Lacteals are branches of the lymphatic system which form a network 

 about the digestive organs. 



