THE VASOMOTOR NERVES. 547 



medulla, but not, so far as is known, from a common nucleus. 

 Intimately connected with the question of the existence of a general 

 vasodilator center is the possibility of definite reflex stimulation 

 of the vasodilator fibers. As stated above, reflex dilatation of the 

 blood-vessels may be produced by stimulating various sensory nerve 

 trunks. The depressor nerve fibers of the heart give only this 

 effect, and the sensory fibers from certain other regions, notably 

 the middle ear and the testis, cause mainly, if not exclusively, a fall 

 of arterial pressure due presumably to vascular dilatation. The 

 sensory nerves of the trunk and limbs, when stimulated by the gross 

 methods of the laboratory, give either reflex vasoconstriction or 

 reflex vasodilatation, and, as was stated above, there is reason to 

 believe that these trunks contain two kinds of sensory fibers, the 

 pressor and the depressor. The action of the former predominates 

 usually, but in deep anesthesia, and particularly in those conditions 

 of exposure and exhaustion that precede the appearance of " shock," 

 the depressor effect is most marked, or indeed may be the only one 

 obtained. To explain such depressor effects we have two possible 

 theories. They may be due to reflex excitation of the centers 

 giving origin to the vasodilator fibers, or to reflex inhibition of the 

 tonic activity of the vasoconstrictor centers. The latter explana- 

 tion is the one usually given, especially for the typical and perhaps 

 special effect of the depressor nerve of the heart. This explanation 

 seems justified by the general consideration that in the two great 

 vascular areas through whose variations in capacity the blood-flow 

 is chiefly regulated, namely, the abdominal viscera and the skin, 

 the vasoconstrictor fibers are chiefly in evidence, and are, moreover, 

 in constant tonic activity. On the other hand, the fact that vaso- 

 dilator fibers exist is presumptive evidence that they are stimulated 

 reflexly, since it is by this means only that they can normally affect 

 the blood-vessels. So that some of the many depressor effects 

 occurring in the body must be due to reflex stimulation of the dila- 

 tors and others to reflex inhibition of the constrictors. It would be 

 convenient to retain the name depressor for the sensory fibers caus- 

 ing the latter effect, and to designate those of the former class by a 

 different name, such as reflex vasodilator fibers.* Only experi- 

 mental 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 vaso- 

 dilator 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 



