VASOMOTOR SUPPLY OF THE ORGANS. 



633 



this pressure state that it is the same as the venous pressure within 

 the sinuses. This we can' understand when we remember the close 

 relations between the subarachnoidal liquid and the large veins 

 and sinuses. We may consider that the large veins are surrounded 

 by the cerebrospinal liquid, and consequently an equilibrium of 

 pressure may be established between them; any rise in the intra- 

 cranial pressure raises venous pressure by compression of the 

 veins, and probably by accelerating the flow of liquid from the 

 Subarachnoidal space into the venous circulation. On the other 

 hand, an increase in venous pressure might be assumed to cause 

 a corresponding rise in intracranial pressure due to the compression 

 following the expansion of the venous walls, and to the retardation 

 of the inflow of cerebrospinal liquid into the veins. If the cerebro- 

 spinal liquid is formed in the choroid plexuses by a process of 

 secretion there may be, of course, a secretion pressure not depend- 

 ent on the mechanical conditions in the vascular circulation, which 

 may cause independent variations in the pressure within the sub- 

 arachnoidal space. But it is 

 not at all certain that the 

 cerebrospinal liquid is formed 

 by a process of active secre- 

 tion, and in any event the 

 changes in mechanical press- 

 ure within the cerebral veins 

 must influence directly the 

 pressure in the surrounding 

 cerebrospinal liquid, and vice 

 versa in the way described 

 above. Compression of the 

 veins of the neck raises the 

 pressure in the cerebral veins 

 and also intracranial pressure, 

 and a higher general arterial 

 pressure also results finally in 

 a higher pressure in the cere- 

 bral veins, and, therefore, in 

 the subarachnoidal space. 

 Under pathological condi- 

 tions rf such as tumors, ab- 

 scesses, excessive formation of 

 cerebrospinal liquid, etc., 

 which lead to a general com- 

 pression of the brain, intra- 

 cranial pressure may be in- 

 creased beyond normal limits. Experimental investigations show 

 that so long as the intracranial pressure remains below that of the 



I 



V 



Fig. 261. 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 filled 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. 



