218 THE CIRCULATION OF THE BLOOD 



only vaso-constrictors, but also vaso-dilator nerves. Vaso-dilator nerves 

 have been described for most parts of the body. In general they are dis- 

 tributed in the same nerve trunks which bear the vaso-constrictors. 



It is not supposed that the vaso-dilators produce widening of the arterioles 

 by stimulation to active muscular contraction; in fact the circular arrange- 

 ment of the muscle fibers would seem to exclude such a deduction. It is 

 probable that there is local inhibition of the tonic contraction of the muscles, 

 thus allowing the mechanical factor of the general blood pressure to dilate 



FIG. 207. Plethysmogram of the Hind Limb of a Cat, Showing Vaso-dilatation upon Stimulating 

 the Sciatic Once per Second. To be read from right to left. (Bowditch and Warren.) 



the vessels. The vaso-dilator nerves are characterized by their response 

 to slowly developed stimuli, shown by Bowditch and Warren, and by the 

 retention of irritability after degeneration of the constrictors has taken place, 

 see figure 207. 



Vaso-dilator Centers. No distinct medullary center has yet been 

 shown to regulate the vaso-dilator nerve activity. Such centers, if they 

 exist, should be influenced by isolating them from their efferent paths, on 

 the one hand, or by stimulation by afferent channels, on the other. The 

 former method of study has revealed nothing that can be compared to the 

 tonic activity of the constrictor center. Efferent dilator-nerve impulses can 

 be reflexly 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 centers in this portion of the spinal cord. 



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, 

 which on first thought might be regarded as vaso-dilator reflexes. But each 

 of these instances can be just as readily explained as inhibitions of the vaso- 

 constrictor tonic activity. This double explanation can, as a matter of fact, 



