376 I'RINCIPLES OF GENERAL PHYSIOLOGY 



centres are responsible for the myenteric reflex, or "law of the intestine." This 

 consists in the production of a relaxation, with inhibition of movements, below 

 the spot at which a mass of food is found, and an increase of tone, together with 

 more powerful contractions above the spot, thus moving onwards the contents 

 of the intestine at this spot. This reflex can be prevented by a set of nerve 

 fibres, in the splanchnic nerves, arising from the central nervous system. Another 

 set of fibres, in the vagus nerve, produces increased movements. In this way, 

 all kinds of movements are provided for. 



The normal movements can be best investigated by the aid of the Riintgen 

 rays. 



The secretion of the various digestive juices is excited in two ways chemical, 

 by the presence of a substance in the blood which has been produced by the action 

 of something contained in the food mass, when it arrives in a particular section of 

 the alimentary canal ; or nervous, by reflexes excited by the sight, smell, or taste 

 of food. The relative part played by these two mechanisms changes from the 

 mouth to the large intestine in such a way that the glands nearer the head are 

 more under control of nervous reflexes. It is doubtful whether secretory nerves 

 play any important part, in normal conditions, in the cases of the pancreas, liver, 

 and small intestine. 



The food is first disintegrated mechanically, sometimes with the aid of bacteria. 



Carbohydrates are then converted into their constituent hexoses or pentoses 

 by a series of enzymes, contained in saliva, pancreatic juice, and succus entericus. 

 Cellulose is usually converted into glucose by the action of bacteria in the large 

 intestine and is probably absorbed in this stage before the bacteria have converted 

 much of it into more degraded products, such as hydrogen or marsh gas. In a few 

 animals, an enzyme capable of hydrolysing cellulose has been found in the 

 secretion of digestive glands. 



Sugars are chiefly absorbed in the small intestine ; when arising from cellulose, 

 in the large intestine. 



Proteins are converted first into proteoses and peptones (higher polypeptides) 

 by the pepsin of the gastric juice, and these into amino-acids and some di-peptides 

 by the trypsin of the pancreatic juice, and finally completely into amino-acids by 

 the erepsin of the succus entericus. 



Proteins are not absorbed in the stomach, but their absorption is practically 

 complete by the end of the small intestine. 



Fats are hydrolysed in the small intestine by the pancreatic juice and absorbed 

 as glycerol and fatty acids, the latter for the most part in solution in bile. In the 

 epithelium of the villi they are resynthesised to neutral fats, which pass into the 

 lymph of the lacteal system and thence to the blood in a finely emulsified state. 



Water and salts are absorbed by the mucous membrane of the intestine. 

 Active intervention on the part of cell mechanisms must be postulated to account 

 for the absorption of solutions isotonic with the blood. 



LITERATURE 



Digestive Mechanisms in General. 

 Biedermann (1911). 



Movements. 



Cannon (1911). 



Secretion of Juices. 

 Pavlov (1901). 



The Stomach. 



Beaumont (1833). 



The Pancreas. 



Bernard (1856). Corvisart (1857-1863). 



