THE CIRCULATION 267 



arterioles in the part from which it comes, and a constric- 

 tion of the arterioles throughout the rest of the body. If 

 a sapid substance, such as pepper, be put in the mouth, 

 the buccal mucous membrane and the salivary glands 

 become engorged, while there is a constriction of the 

 arterioles throughout the body. The vaso-dilator central 

 mechanism is not general in its action like the vaso- 

 constrictor, but is specially related to the different parts of 

 the body. 



This is a matter of the greatest importance in physiology 

 and pathology. It explains the increased vascularity of a 

 part when active growth is going on. The changes in the part, 

 or the products of these, stimulate the afferent nerve. This 

 reflexly stimulates the vaso-dilator mechanism of the part, 

 and thus causes a free flow of blood into the capillaries, and 

 at the same time, by causing a general constriction of the 

 arterioles, maintains or actually raises the arterial pressure, 

 and thus forces more blood to the situation in which it is 

 required. 



The same process occurs in the case of the stomach during 

 digestion, in the case of the kidney during secretion, and 

 in the process of inflammation. 



Not only does peripheral stimulation act in this way, but 

 various states of the brain, accompanied by emotions, may 

 stimulate part of the vaso-dilator mechanism, as in the act 

 of blushing. 



Again, it has been shown that stimulation of the central 

 end of the depressor nerve (superior cardiac branch of the 

 vagus) causes a dilatation of the arterioles chiefly in the 

 abdominal cavity, but also throughout the body generally. 

 This is the most generalised vaso-dilator reflex known 

 (see p. 240). 



(b) Position. While the dominant vaso-constrictor centre 

 is in the medulla, the vaso-dilator centres seem to be dis- 

 tributed throughout the medulla and spinal cord. 



III. CAPILLARY PRESSURE. 



It has already been shown that this is less than in the 

 arteries and greater than in the veins. 



