602 CIRCULATION OF BLOOD AND LYMPH. 



length of 3 mms., a breadth of 1 to 1.5 mms., and hes 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, it is 

 customary to use in explaining the various vasomotor phenomena 

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

 which indicate the possibility that the veins and capillaries 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 

 capacity of the capillary and venous areas. 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, 

 on the contrary, which when stimulated cause a diminution in 



