194 STUDIES IN ADVANCED PHYSIOLOGY. 



nerves are of two kinds; first, vaso-motor nerves which 

 when stimulated cause the arteries to contract the vaso- 

 constrictor nerves; second, vaso-motor nerves which when 

 stimulated cause the arteries to dilate vaso-dilator nerves. 



1. Vaso-consttictor nerves. The vaso-constrictor nerve 

 fibres are found in almost every part of the body. They 

 seem in a continued state of excitation, keeping the arteries 

 to which they go constantly contracted, for when such a 

 nerve is cut the arteries affected at once dilate. The ex- 

 planation of this is found in the fact that the cutting of the 

 vaso-constrictor nerves puts an end to the tonic influence 

 which they possess and the arteries freed from their con- 

 trol expand to their natural dimensions. Why the arteries 

 should be kept continually contracted is not hard to under- 

 stand. There are times when certain organs of the body 

 in order to do special work need an extra supply of blood. 

 The stomach needs more blood during digestion than when 

 idle between meals. Now, by keeping the gastric arteries 

 in a tonic or continued contraction they may be dilated by 

 relaxing the muscles when the process of digestion begins. 

 As arteries cannot forcibly expand (muscle fibres can never 

 be made to expand forcibly) , they must be contracted reg- 

 ularly in order to make an expansion when desired pos- 

 sible. While cutting a constrictor nerve causes a dila- 

 tation in the vessels affected because, as already stated, 

 the tonic control is cut off, a stimulation of the nerve 

 causes an increased contraction in the artery supplied. If 

 in a rabbit the constrictor nerve going to the transparent 

 ear be stimulated, the ear flap at once becomes pale. 



Distribution of Faso- Constrictor Nerves. 



The course of these constrictor nerves throughout the 

 body is about as follows: First. In the head they arise in 

 the medulla, pass from this into the sympathetic ganglia of 

 the neck, from which they proceed to all parts of the head, 

 usually running along with the cranial nerves. The tri- 

 geminal nerve, especially, is rich in such constrictor fibres. 



