CIRCULATION 281 



play the same part in regard to the muscular fibres of the 

 arterioles as the inferior cardiac branch of the vagus does 

 in regard to the cardiac muscular fibres. As examples of 

 vaso-dilator nerves the gastric branches of the vagus carrying 

 vaso-dilator fibres to the mucous membrane of the stomach, 

 and the nervi erigentes carrying vaso-dilator fibres to the 

 external genitals, may be taken. 



The vaso-dilator nerves of most parts of the body run side 

 by side with the vaso-constrictor nerves; and, hence, curious 

 results are often obtained. If the sciatic nerve of a dog be 

 cut, the arterioles of the foot dilate. If the peripheral end 

 of the cut nerve is stimulated, the vessels contract. But, 

 after a few days, if the nerve be prevented from uniting, the 

 arterioles of the foot recover their tonic contraction, and, if 

 the sciatic nerve is then stimulated, a dilatation, and not a 

 constriction, is brought about. The vaso-constrictor fibres seem 

 to die more rapidly than the vaso-dilator fibres which run 

 alongside of them. Under certain conditions the activity of 

 the vaso-dilator fibres seems to be increased. Thus, if, when 

 the limb is warm, the sciatic nerve is stimulated, dilatation 

 rather than constriction may occur. Again, while rapidly re- 

 peated and strong induction shocks are apt to cause constric- 

 tion, slower and weaker stimuli tend to produce dilatation. 



Course. The vaso-dilator nerves pass out chiefly by the 

 anterior roots of the various spinal nerves, and do not pass 

 through the sympathetic ganglia, but run as medullated 

 fibres to their terminal ganglia (fig. 130). Bayliss has recently 

 shown that the vaso-dilator fibres for the hind limb leave the 

 cord by the posterior roots, and that they are connected with 

 the cells in the ganglia. 



3. Portions of Nervous System Presiding over the Vaso- 

 motor Mechanism. Since a set of nerves causing constriction 

 of the arterioles, and another set causing dilatation exist, 

 we must conclude that there are two mechanisms in the 

 central nervous system one a vaso-constrictor, the other a 

 vaso-dilator. 



A. Vaso-constrictor Centre. (a) Mode of Action. This 

 mechanism is constantly in action, maintaining the tonic 

 contraction of the arterioles. 



