THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 227 



circulation is similar to that operating when it is stored; 

 it is a combination of nervous action and an internal secre- 

 tion. Just above each kidney is an organ about the size of 

 the end of the thumb, the adrenal gland. The mid-portion 

 of this gland is called the medulla, the outer portion, the 

 cortex. The cells of the adrenal medulla are controlled 

 by sympathetic nerves, i.e., nerves which are excited by 

 exposure to cold, by strenuous muscular efforts and by 

 strong emotions, such as fear and rage. The substance 

 which these cells pour into the blood stream, when they are 

 stimulated by the nerve impulses, has the remarkable power 

 of producing in all parts of the body the same effects that 

 are produced by the sympathetic nerve impulses them- 

 selves. These impulses not only cause a discharge of adrenin 

 into the circulating blood, they also call forth glucose from 

 the stores in the hver. But the secreted adrenin can do 

 hkewise. Thus the sympathetic impulses and the secreted 

 adrenin cooperate to mobilize the reserves of energy-yielding 

 material when the cells are hkely to be in need of it. 



Fat is stored in fat cells, and there is evidence that protein 

 is stored in Hver cells. After a considerable period of fasting 

 the blood still has a normal fat and protein content; the 

 fat and protein reserves have been reduced, however, 

 and yet organs of prime importance, e.g. the heart and the 

 brain, show no signs of any weakness or decrease of weight. 

 They are maintained at the expense of the reserves and 

 of less important structures. The conditions which govern 

 the laying-by of fat and protein reserves and which bring 

 them out for use when they are needed are still largely 

 unknown. 



The Constancy of the Water Content oj the Blood. As we 

 have seen, the blood plasma and the tissue fluids are watery 

 solutions of salts, sugar and albuminous materials. The 

 sap of cells is also a watery solution of hke materials. Between 

 this sap and the tissue fluid everywhere is the cell membrane 

 through which water and certain of the dissolved substances 

 can readily pass. Ordinarily the water of the plasma is 

 balanced against that in the tissue fluid, and that in turn 

 is balanced against the water in the cells. If the water of the 

 plasma is increased it disturbs the balance, and, when 



