THE VASOMOTOR NERVES. 615 



length of 3 mms., a breadth of 1 to 1.5 mms., and lies about 2 

 to 2.5 mms. lateral to the mid-line. Assuming the existence of 

 this group of cells, we must attribute to them functions of the first 

 importance. Like other motor cells, they are capable of being 

 stimulated reflexly and by this means the regulation of the blood- 

 flow is largely controlled. Moreover, they are in constant activity, 

 due doubtless also to a constant reflex stimulus from the inflow 

 of afferent impulses. The complete loss of this tonic influence 

 would result in a complete vascular paralysis, the small arteries 

 would be dilated, peripheral resistance would be greatly diminished, 

 and the arterial pressure in the aorta would fall from a level of 

 100-150 mms. Hg to about 20 or 30 mms. Hg, a pressure insuffi- 

 cient to maintain the life of the organism. We must conceive, 

 also, that in this vasoconstrictor center the different cells are con- 

 nected by definite paths with the vasoconstrictor fibers to the 

 different regions of the body; that some of the cells, for instance, 

 control the activity of the fibers distributed to the intestinal area, 

 and others govern the vessels of the skin. Under physiological 

 conditions the different parts of the center may, of course, be acted 

 upon separately. In this description of the vasoconstrictor center 

 and the efferent vasoconstrictor fibers reference has been made only 

 to their action on the small arteries. This is, generally speaking, 

 the apparatus, which on the basis of experimental work, we are 

 justified in using to explain the various vasomotor phenomena of 

 the body. There are, however, many observations on record 

 which indicate the possibility that the veins also may be supplied 

 with motor nerve fibers, and that reflex control of the distribution 

 of the blood and the regulation of the return flow to the heart may 

 be exerted in part through changes in the caliber of the venous 

 vessels. Unfortunately, our knowledge upon this point is too in- 

 complete at present to be used with confidence in the explanation 

 of specific phenomena. It is, nevertheless, a possibility that must 

 be held in mind. Some of the more significant facts bearing upon 

 this point are referred to below. 



Vasoconstrictor Reflexes Pressor and Depressor Nerve 

 Fibers. It is obvious that such a mechanism as that described 

 above is susceptible of reflex stimulation through sensory nerves, 

 and according to our general knowledge we should suppose that 

 a tonic center of this kind may have its tonicity increased (excita- 

 tion) or decreased (inhibition). Numerous experiments in phys- 

 iology warrant the view that both kinds of effect take place 

 normally. Those afferent nerve fibers which when stimulated 

 cause reflexly an excitation of the vasoconstrictor center, and 

 therefore a peripheral vasoconstriction and rise of arterial pressure, 

 are frequently designated as pressor fibers, or their effect upon the 

 circulation is designated as a pressor effect. Those afferent fibers, 



