THE BRAIN 239 



pressure by increasing the tonus of the muscles of the 

 vessels. 



The cardie-accelerating centre is also supposed to be 

 tonic. 



In general, the centres for the organs of circulation bring 

 about many reflex actions. This is clearly apparent from 

 the many and various actions by which these centres regu- 

 late the distribution of blood according to the needs of the 

 body. 



3. Centres for certain movements and secretions of the 

 alimentary canal (see Chapters VII and IX). These are: 



(a) Centres for biting, sucking, mastication, deglutition., 

 vomiting i and perhaps also for the movements of tJie stomach 

 and intestines. 



The centres of biting, sucking, and mastication are volun- 

 tarily stimulated by the cerebral hemispheres ; the other 

 centres are not subject to the will. Deglutition takes place 

 reflexly when the food has been pushed from the tongue 

 behind the anterior pillars of the soft palate. ' ' Empty 

 swallowing M is made possible by the swallowing of saliva; 

 without saliva it is impossible. The vomiting centre is not 

 only stimulated reflexly, but can also be stimulated by 

 psychical influence (sight of nauseous objects). 



(b) Centre of salivary secretion, perhaps also for gastric, 

 intestinal, and pancreatic secretions. 



The stimulation of these centres takes place involuntarily, 

 chiefly reflexly by the introduction of food in the alimentary 

 canal ; sometimes also, by psychical influences, sight of 

 tempting food stimulates salivary and gastric secretion. 



4. Centres for the secretion of sweat and tears (see pages 

 109 and 1 10). These centres also are not stimulated volun- 

 tarily. The perspiration centre is directly stimulated by the 

 raising of the temperature (heat) and also by lack of oxygen 

 and the accumulation of carbon dioxide in the blood 

 (asphyxia). Its activity is influenced by psychical condi- 

 tions (sweat of fear). 



The stimulation of the centre for lachrymal secretion takes 



