128 HUMAN PHYSIOLOGY 



centres lie in the medulla, the corpora quadrigemina, and 

 the spinal cord. 



Vomiting is the emptying of the contents of the stomach after 

 the cardiac sphincter has opened. It is brought about mainly by 

 the contraction of the diaphragm and the muscles in the abdominal 

 walls, by which the intra-abdominal pressure is increased to such 

 an extent that vomiting results. To a limited extent the antrum 

 also takes part in vomiting. 



Vomiting is brought about reflexly (stimulation of the sensory 

 nerve in the stomach by abnormal substances) or by drugs which 

 directly affect the vomiting centre in the medulla or by psychical 

 influences (nauseating sights). 



2. Chemistry of gastric digestion. When the food has 

 reached the stomach it is still further subjected to the action 

 of salivary ferments (amylolytic period of gastric digestion) 

 for about half an hour. After this, the secretion of the acid 

 gastric juice stops the action of the ptyalin. Then the 

 action of the gastric juice begins. 



The active constituents of gastric juice are pepsin, hydro- 

 chloric acid, and rennin. The gastric juice digests proteids, 

 inverts cane-sugar, and curdles milk, but does not act upon 

 fats. 



(a) Proteid digestion. Proteid digestion is brought about 

 by the action of free hydrochloric acid and of the pepsin ; by 

 means of these the proteids of the food are split up into 

 albumoses and peptones. 



The role of the hydrochloric acid in proteid digestion is 

 twofold : 



First, it causes the proteid body to swell more or less and 

 thereby facilitates the subsequent action of pepsin. 



Secondly, in connection with pepsin it causes a peculiar 

 splitting of the proteids. 



The hydrochloric acid can by itself split proteids into 

 albumoses and peptones, but to do so the acid must be 

 concentrated, or it must act at boiling temperature or for a 

 long time. Pepsin cannot split up proteids without the aid 

 of hydrochloric acid. 



Perhaps the hydrochloric acid is the real splitting agent of the 



