THE AUTONOMIC NERVOUS SYSTEM 119 



at random is subjected to stimulation. When such a nerve 

 is cut the vessels in the field of its distribution are gener- 

 ally found to dilate; this is taken to show that a tonic 

 influence was being exerted through the vasoconstrictor 

 fibers and that this has ceased with the interruption of their 

 continuity. 



We speak of a vasoconstrictor center in the medulla. 

 There is evidence that impulses are normally flowing at 

 all times from a certain region within this section of the 

 brain to hold the small blood-vessels in most parts of the 

 body in a state of contraction beyond that which is natural 

 to them. Three orders of neurons must be required to 

 accomplish the purpose: first, those which descend from 

 the medulla to end by synapses against preganglionic 

 neurons in the cord; second, these same preganglionic 

 neurons, whose axons run to detached ganglia in the sym- 

 pathetic chains or elsewhere; and third, postganglionic 

 fibers extending from the ganglia to the blood-vessels. 

 Destruction of the well-defined vasoconstrictor center in 

 the medulla results in a universal slackening of the essen- 

 tial vascular tone in the entire body. 



The second variety of vasomotor fibers is not so often 

 seen in action. They are the vasodilators, fibers which, 

 when stimulated, cause the enlargement of the vessels and 

 so of the blood-stream in the parts to which they lead. 

 This action is an inhibition. It was first demonstrated 

 in the case of a small nerve known as the chorda tympani, 

 which makes a connection with one of the salivary glands. 

 Cutting the chorda tympani has little effect on the circu- 

 lation in the gland, but when the nerve is stimulated the 

 gland becomes flushed and swollen. At the same time the 

 outflow through the veins is obviously increased; it is said 

 that the blood coming away no longer shows the darkened 

 color usual to venous blood, but is as bright as the arterial. 

 The experiment has shown that in this nerve at least there 

 are fibers through which an increase of the volume of the 

 local circulation can be brought about. 



There are other, though not many, examples of vaso- 



