THE VASOMOTOR MECHANISM 381 



shown marked loss of vasomotor tone, but even if we consider 

 vasomotor fatigue the primary factor we must grant, of course, 

 that there are numerous additional factors modifying sleep. The 

 condition of the cerebral cells and the nature of their activity 

 doubtless have much to do with the phenomenon. These, how- 

 ever, are factors which physiology at present is unable to analyze 

 completely, so that the vasomotor theory affords our most satisfac- 

 tory explanation of sleep from the physiological standpoint. 



Adrenin. The effect of this hormone upon the vascular system, 

 as stated previously (Chap. XII), is to stimulate the vaso- 

 constrictor fibers at their terminations in the muscles of the ar- 

 terioles. The constant presence of this hormone in the blood is 

 probably an important factor in maintaining that degree of vaso- 

 motor tone upon which the well-being of the Body depends. The 

 great outpouring of adrenin into the blood under emotional stress 

 so much increases the constriction of the blood vessels in the skin 

 and the splanchnic area as to produce a pronounced rise in blood- 

 pressure, with a correspondingly augmented cerebral circulation. 

 The same influence acts to divert the blood largely from these 

 regions to the skeletal muscles. The vessels of these latter being 

 unprovided with vasoconstrictor fibers are not involved in the 

 adrenin effect. Since the brain and the skeletal muscles are the 

 regions specially in need of adequate nourishment in crises the 

 adaptive character of this reaction is obvious. The substance 

 adrenin as used experimentally shows several striking characteris- 

 tics. In the first place a very small concentration of it (one part 

 in ten thousand), introduced into a capillary region, brings about 

 so strong a constriction in the immediate neighborhood as to stop 

 the flow of blood completely through that region. It is possible 

 thus to prevent troublesome bleeding in small operations. The 

 effect of adrenin used in this way is, however, very transient; re- 

 peated injections are necessary to maintain the constricted state. 



The Lymphatics. The living cells of the Body, as previously 

 pointed out (Chap. XVII), are bathed in lymph, a liquid derived 

 from the blood and serving as the intermediary by which inter- 

 changes of food materials, gases and waste substances between 

 it and the cells are carried on. At the same place it was shown 

 that there is a continuous movement of liquid from the blood into 

 the lymph spaces, necessitating a system whereby the accumu- 



