342 THE HUMAN BODY 



should be two distinct means thus provided for securing the same 

 result. As a matter of fact the two mechanisms do not seem to 

 overlap to any great extent; they rather supplement each other. 

 The vasoconstrictor mechanism is confined, in the main, to the 

 blood-vessels of the skin and viscera; the dilator mechanism is 

 distributed chiefly to the muscles, the glands, and the genital 

 organs. 



Through such arrangements the distribution of the blood in the 

 Body at any moment is governed : so that working parts shall have 

 abundance and other parts less, while at the same time the general 

 arterial pressure remains the same on the average; since the ex- 

 pansion of a few small local branches but little influences the total 

 peripheral resistance in the vascular system. Moreover, commonly 

 when one set of organs is at work with its vessels dilated, others 

 are at rest with their arteries comparatively contracted, and so a 

 general average blood-pressure is maintained. Few persons, for ex- 

 ample, feel inclined to do brain-work after a heavy meal ; for then a 

 great part of the blood of the whole Body is led off into the dilated 

 vessels of the digestive organs, and the brain gets a smaller supply. 



The Vasodilator Center. There is reason to believe that the 

 vasodilator nerves are under the control of a center in the medulla, 

 which is in turn subject to the influence of afferent impulses of 

 various sorts. The exact location of this center has not been de- 

 termined. So far as can be judged from observation of vaso- 

 dilator phenomena the vasodilator center is probably not in con- 

 stant tonic activity, as is the constrictor center, but is aroused to 

 activity only when afferent stimuli come to it from certain par- 

 ticular regions. 



The Relation of Vasomotor Tone to Cerebral Activity. The 

 circulation through the brain differs in some important respects 

 from that of the rest of the body. The differences arise from the 

 fact that the brain, a fluid and therefore incompressible mass, is 

 inclosed in an unyielding receptacle, the skull, which it fills com- 

 pletely. The result is that the cerebral blood-vessels occupy their 

 allotted space, which cannot be either increased or diminished ap- 

 preciably. The total volume of blood in the brain at any time is 

 therefore practically constant, and the circulation through the 

 brain can only be altered by changing the rate at which the blood 

 flows through it. In such an arrangement as this, where local 



