VASOMOTOR SUPPLY OF THE ORGANS. 



557 



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 

 communicated to the outside liquid, which, in turn, will compress the veins 

 correspondingly. The expansion on the arterial side is made possible by a 

 corresponding diminution on the 



venous side where 

 pressure is least. 



the internal 



The recorded measure- 

 ments 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 Hill * 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 f have proved ex- 

 perimentally that a rise of 

 general arterial pressure is fol- 

 lowed, 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 simply 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 important thing is the increase in the quantity 



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



t See Gartner and Wagner, " Wiener med. Wochenschrift," 1887; de 

 Boeck and Verhorgen, "Journal de Medecine, etc.," Brussels; Roy and Sher- 

 rington, "Journal of Physiology," 11, 85, 1890; Reiner and Schnitzler, 

 "Archiv f. exp. Pathol, u. Pharmakol. , " 38, 249, 1897. 



Fig. 232. Schema to represent the 

 transmission of arterial pressure through 

 the brain substance to the veins: A, The 

 artery, V, the vein, represented as entering 

 into and emerging from a box with rigid 

 walls and fijled with incompressible liquid; 

 c, c, the intervening area of small arte- 

 ries, etc. An expansion of the walls of 

 the arterial system by the pulse wave or by 

 a rise of arterial pressure increases the pres- 

 sure on the surrounding liquid and this is 

 transmitted through the liquid to the walls 

 of the veins and compresses them, since at 

 this point of the circuit the intravascular 

 pressure is low. 



