VASOMOTOR SUPPLY OF THE ORGANS. 



629 



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 



I 



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. 



a higher pressure in the cere- 

 bral veins, and, therefore, in 

 the subarachnoidal space. 

 Under pathological condi- 

 tions, 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 

 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 



