VASO-DILATOR REFLEXES 



251 



tivity of the constrictor center. Efferent dilator-nerve impulses can be re- 

 flexly produced by sensory stimulation. The isolated lumbar cord of a dog 

 is capable of reflex vaso-dilator activity, since stimulation of the skin of 

 the penis leads to reflex vaso-dilatation, indicating the presence of local 

 vaso-dilator reflex mechanisms or paths through the lumbo-sacral portion 

 of the spinal cord. 



TABLE SHOWING THE INFLUENCE OF RATE AND STRENGTH OF STIMULATION OF THE 

 SCIATIC NERVE ON THE BLOOD VESSELS OF THE LEG (BOWDITCH AND WARREN) 



Vaso-dilator Reflexes. Perhaps the only unquestioned case of reflex 

 vaso-dilatation is that of the lumbar cord just mentioned. It is true that 

 many apparent reflexes can be noted, for example the increased flow of 

 blood in the salivary glands under gustatory reflexes, the blushing of the 

 skin on exposure to sudden warmth, or even the blushing of emotional 

 origin. These on first thought would be regarded as vaso-dilator reflexes. 

 In all these cases there is a widening of the peripheral arterioles with a 

 great increase in the volume of blood flowing through the local vascular 

 bed. But each instance can be just as readily explained as inhibition 

 of the vaso-constrictor tonic activity. This double explanation can, as a 

 matter of fact, be applied to the action of the depressor nerve described 

 above, page 248. However, the confusion is in part due to the diffi- 

 culty of analyzing the two classes of nerves in the same nerve trunk. 

 All the thoracic spinal nerves and the upper lumbar trunks contain both 

 vaso-constrictor and vaso-dilator nerves. The usual methods of physio- 

 logical stimulation with rapidly interrupted currents we now know are 

 normal stimuli for the constrictors only. Stimulations at the rate of one 

 or two per second call forth responses in the dilator fibers. Also, the vaso- 

 dilator fibers degenerate more slowly and retain their irritability longer 

 than the constrictor fibers when both are isolated from their cell bodies 

 by sectioning. The method of slow stimulation and the method of differ- 

 ential degeneration were given us by Bowditch and Warren. 



