THE CIRCULATION THROUGH SPECIAL ORGANS 443 



actually receives more blood during this period of heightened arterial 

 blood pressure, and hence, it must be concluded that this reaction is 

 of nervous origin. It seems that the increased intracranial pressure 

 influences the cerebral centers directly and gives rise to a general reflex 

 vasoconstriction which becomes associated later on with an inhibition 

 of the cardiac and respiratory activities. It might be mentioned that 

 the procedure just described may be used to imitate the chain of 

 symptoms generally associated with certain lesions of the brain and 

 fractures of the skull. 



The Regulation of the Cerebral Blood Supply. — It has always been 

 held that the vascularity of the brain is determined exclusively by 

 indirect factors, such as vasomotor reactions in other parts of the body 

 and gravity. At the present time, however, when the existence of 

 vasomotor nerves to the cerebral blood-vessels can no longer be doubted, 

 this purely mechanical conception must be modified somewhat to 

 conform to actual conditions. Quite naturally, the extracranial 

 factors just mentioned cannot be disregarded entirely, because it seems 

 certain that they are capable at times of exerting an influence which 

 is not inferior to that of the intracranial vasomotors. Thus, it may be 

 stated that the vascularity of the cerebrum stnd neighboring structures 

 is controlled in a direct and an indirect way, first by the vasomotor 

 changes inside the cranial cavity and secondly, by vasomotor and 

 other changes in more remote parts of the body. 



This conclusion, however, need not defer us from briefly discussing 

 the older view of Roy and Sherrington, which contends that the vas- 

 cularity of the brain is controlled solely in an indirect way by vaso- 

 motor reactions occurring in other parts of the body. The claim is 

 made that two circuits are chiefly concerned in this interchange, 

 •namely, the portal and the cutaneous. It is readily conceivable that 

 !a dilatation occurring in one or both of these vascular areas must lead 

 to a withdrawal of a certain quantity of blood from the cerebral blood- 

 vessels. Contrariwise, it may be inferred that a constriction in either 

 ^region must force a certain quantity of blood into the cerebral circuits, 

 tin substantiation of this view, Mosso^ has shown that a constric- 

 [tion of the blood-vessels of the legs is always associated with an in- 

 ;crease in the volume of the brain. These observations were made 

 'upon men with trephine openings in the skull, their limbs having been 

 enclosed in a plethysmograph. Quite similarly, it has been observed 

 by this author that the volume of the brain is diminished during sleep, 

 while that of the limbs is increased. A constriction of the blood-vessels 

 of the posterior extremities takes place whenever the mental activity 

 :is heightened and especially during emotional states. We have seen 

 above that the portal organs require an increased amount of blood 

 during digestion and that this extra supply of blood can only be 

 obtained by withdrawing it from other parts of the body, inclusive 

 of the cerebrum. Concurrently, it may be gathered that the cerebrum 



1 Mosso, Ueber den Kreislauf des Blutes im mensch. Gehirn, Leipzig, 1881. 



