DISCOVERY OF THE VASO-MOTOR NERVES 



2 33 



before dilated return to their natural size, and the parts regain their former 

 condition. And, besides, if the stimulus is very strong or very long-continued, 

 the amount of normal constriction is passed and the vessels become much 

 more contracted than before. The natural condition, which is midway 

 between extreme contraction and extreme dilatation, is called the natural 

 tone of an artery. If this is not maintained, the vessel is said to have lost 

 tone, or, if it is exaggerated, the tone is said to be too great. The effects 

 described as having been produced by section of the cervical sympathetic 

 and by subsequent stimulation are not peculiar to that nerve and the vessels 

 to which it is distributed. 



FIG. 201. Small Artery and Vein of the Frog's Web. A, Under normal conditions; 

 B, upon stimulation of the sciatic nerve; Ar, artery; V, vein. In this experiment the vein 

 also showed well-marked vaso-constriction. (Greene.) 



It has been found that for every part of the body there exists a nerve the 

 division of which produces the same effects, viz., dilatation of the vessels. 

 Such may be cited as the case with the sciatic, the splanchnic nerves, and 

 the nerves of the brachial plexus; when these are divided, dilatation of the 

 blood vessels in the parts supplied by them takes place. It appears, 

 therefore, that nerves exist which have a distinct control over the vascular 

 supply of every part of the body. These are called vaso-motor or vaso- 

 constrictor nerves. Bnt the arterioles are also under the influence of a 

 second set of nerves, also discovered by Claude Bernard, which produce 

 -exactly the opposite influence, i.e., dilatation. These nerves are called vaso- 

 dilator nerves. 



