THE VASOMOTOR NERVES. 623 



effects occurring in the body must be due, therefore, to reflex 

 stimulation of the dilators and others to reflex inhibition of the 

 constrictors. It would be convenient to retain the name depressor 

 for the sensory fibers causing the latter effect, and to designate 

 those of the former class by a different name, such as reflex vaso- 

 dilator fibers.* Only experimental work can determine positively 

 to which effect any given reflex dilatation is due, but provisionally 

 at least it would seem justifiable to assume that dilatation by reflex 

 stimulation of the vasodilator fibers occurs in those parts of the 

 body in which vasodilator fibers are known to exist. Thus, the 

 erection of the penis from stimulation of the glans may be explained 

 in this way, also the congestion of the salivary glands during activity, 

 the blushing of the face from emotions, and possibly the dilatation 

 in the skeletal muscles during contraction. Gaskell and others 

 have given reasons for believing that the vessels in the muscles are 

 supplied with vasodilator nerve fibers, and Kleen f has shown that 

 mechanical stimulation of the muscles kneading, massage, etc. 

 causes a fall of arterial pressure. 



Vasodilatation Due to Afferent Fibers. The existence of definite effer- 

 ent vasodilator fibers in the nerve trunks to the limbs has been made doubt- 

 ful by the work of Bayliss. This author has discovered certain facts which at 

 present tend to make the question of vasodilatation more obscure, but which, 

 when fully understood, will doubtless give us a much deeper insight into the 

 subject. Briefly stated, he has shownj that stimulation of the posterior roots 

 of the nerves supplying the lumbo-sacral and the brachial plexus causes vas- 

 cular dilatation in the corresponding limbs. He has given reasons for believing 

 that the fibers involved are afferent fibers from the limbs and that, therefore, 

 when stimulated they must conduct the impulses in a direction opposite to 

 the normal antidromic. It is most difficult to understand how such 

 impulses, conveyed to the terminations of the sensory fibers, can affect the 

 muscular tissue of the blood-vessels. It is most difficult to understand also 

 how such anatomically afferent fibers can be stimulated reflexly in the cen- 

 tral nervous system. Some light has been thrown upon this subject by recent 

 work upon the vascular dilatation of the conjunctival membrane when 

 irritated locally by substances, such as oil of mustard, that cause inflammation. 

 It has been found that the vascular dilatation in these cases is not due to a 

 direct effect of the irritant on the vessels, and that it is not a reflex effect 

 through the spinal cord or the posterior root ganglia. On the other hand, the 

 effect is not obtained if the terminations of the sensory fibers of the region are 

 anesthetized, or if the sensory nerves to the region are cut and time is allowed 

 for them to degenerate. The hypothesis used to explain the reaction is that the 

 sensory fibers to the region branch at their termination, one branch going to 

 the sensory ending, the other to the blood-vessels. When, therefore, the sen- 

 sory ending is stimulated there is a local reflex, of the nature of an axon-reflex, 

 through the other limb of the bifurcation, which effects a dilatation of the 

 blood-vessel. If this explanation holds upon further examination, it will 

 establish the existence of a peripheral mechanism for the production of local 

 vasodilatation which has not been taken into account heretofore (see p. 152). 



* See Hunt, "Journal of Physiology," 18, 381, 1895. 

 t Kleen, "Skandinavisches Archiv f. Physiologic," 247, 1887. 

 t Bayliss, "Journal of Physiology," 26, 173, 1900, and 28, 276, 1902. 

 Ninian Bruce, "Quarterly Journal of Exp. Physiology," 6, 339, 1913; 

 also Bardy, "Skandinavisches Archiv f. Physiologic," 32, 198, 1914. 



