THE VASOMOTOR NERVES. 567 



to believe that they are ail normally connected with a single group 

 of nerve cells located in a definite part of the nervous system. The 

 dilator fibers in the facial, glossopharyngeal, and cervical sympa- 

 thetic (distributed through the trigeminal) all arise probably in the 

 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 ppssibility of definite reflex stimulation 

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

 blood-vessels may be produced by stimulating various nerve trunks 

 containing afferent fibers. 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 afferent fibers, the pressor and the depressor. The action of the 

 former predominates usually, but in deep anesthesia, and particu- 

 larly 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 explanation 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 vasodilator 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. Some of the many depressor 

 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 

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



