634 CIRCULATION OF BLOOD AND LYMPH. 



arteries supplying the brain the circulation through the brain is 

 not markedly affected. If, however, the intracranial pressure rises 

 above general arterial pressure the flow through the substance of 

 the brain is prevented and a condition of anemia results which 

 would presumably cause unconsciousness. In anesthetized ani- 

 mals submitted to such a condition it has been shown that a 

 compensation takes place; the anemic condition of the medulla 

 stimulates the cardio-inhibitory center, causing a slower heart 

 beat; at the same time it stimulates also the vascmotor center, 

 causing a general vasoconstriction in the rest of the body, the 

 result of which is to raise the arterial pressure and reestablish the 

 cranial circulation (Gushing).* 



Reduced to its simplest form, the normal conditions may be represented 

 by a schema such as is given in Fig. 261. A system with an artery, capillary 

 area, and a vein is represented as inclosed in a rigid box and surrounded by 

 an incompressible liquid. According to the conditions prevailing in the body, 

 the pressure in the interior of A and its branches is much higher than in V. 

 If, now, the pressure in A is increased the greater pressure brought to bear on 

 the walls will tend to expand them; a greater pressure will thereby be com- 

 municated to the outside liquid, which, in turn, will compress the veins cor- 

 respondingly. The expansion on the arterial side is made possible by a corre- 

 sponding diminution on the venous side where the internal pressure is least. 



The recorded measurements of the intracranial pressure show 

 that it may vary from 50 to 60 mms. of mercury, obtained during 

 the great rise of pressure following strychnin poisoning, to zero 

 or less, as obtained by Hillf from a man while in the erect pos- 

 ture. In this position the negative influence of gravity is at its 

 maximum. 



The Effect of Variations in Arterial Pressure upon the Blood- 

 flow through the Brain. Quite a number of observers! have proved 

 experimentally that a rise of general pressure is followed not only 

 by an increase in the intracranial tension, but also by an increased 

 blood-flow through the brain. There has been much discussion as 

 to whether a rise of arterial pressure in the basilar arteries can cause 

 any actual increase in the amount of blood in the brain or whether 

 it expresses itself solely or mainly as an increased amount of flow. 

 In the other organs of the body, except perhaps the bones, a general 

 rise of pressure, not accompanied by a constriction of the organ's 

 own arteries, causes a dilatation or congestion of the organ together 

 with an increased blood-flow. Physiologically the congestion 

 that is, the increased capacity of the vessels is of no value; the 



* Gushing, "American Journal of the Medical Sciences," 1902 and 1903, 

 and also Eyster, Burrows and Essick, "Journal of Experimental Medicine," 

 11, 489, 1909. 



t Bayliss and Hill, "Journal of Physiology," 18, 356, 1895. 



* See Gartner and Wagner, "Weiner med. Wochenschrift," 1887; de Boeck 

 and Verhorgen, "Journal de Me"decine, etc.," Brussels; Roy and Sherrington, 

 "Journal of Physiology," 11, 85, 1890; Reiner and Schnitzler, "Archiv f. exp. 

 Pathol. u. Pharmakol.," 3, 249, 1897. 



