240 THE CIRCULATION OF THE BLOOD 



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 ac- 

 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 centers in this portion of the spinal cord. 



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.) 



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 ex- 

 posure to sudden warmth, or even the blushing of emotional origin, these 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, be applied 

 to the action of the depressor nerve described above, page 237. On the 

 whole, however, while we cannot directly and unquestionably prove the fact, 

 yet it is probable that each of the above examples may be accepted as examples 

 of reflex vaso-dilatation by direct action on a vaso-dilator center or centers in 

 the cord. 



The Relation of Vaso-constrictor and Vaso-dilator Activity. The 

 distribution of two sets of regulative fibers for the muscular walls of the 

 blood vessels, when considered in connection with the other factors of the 

 vascular apparatus, gives a wonderfully complete mechanism for the co-ordi- 

 nation of the vascular supply with the activity of the different organs. Gen- 

 eral and broadly distributed activity of the constrictors produces increase of 



