540 CIRCULATION OF BLOOD AND LYMPH. 



to his description, the center is bilateral, that is, consists of a 

 group of cells on each side, and lies about the middle of the fourth 

 ventricle in the tegmental region, in the neighborhood of the nucleus 

 of the facial and of the superior olivary. In the rabbit it has a 

 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 sensory 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. There seems to be no 

 question now that in those conditions known as surgical shock the 

 loss of control by the vasomotor center, and the consequent vascular 

 paralysis and fall of blood-pressure, are the chief conditions of a 

 serious character. We must conceive, also, that in this vasocon- 

 strictor center the different cells are connected 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. 



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 effects 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 

 the tone of the vasoconstrictor center and therefore a periph- 

 eral vasodilatation and fall of arterial pressure, are designated as 

 depressor nerve fibers, or their effect upon the circulation is a de- 

 pressor effect. Pressor effects may be obtained by stimulation of 

 almost any of the large nerves containing afferent fibers, but espe- 



