258 THE CIRCULATION OF THE BLOOD 



given moment can undergo no considerable change. Provision for more 

 or less blood must therefore be afforded by changes in the velocity of 

 now. 



Physiological Conditions of the Intracranial Circulation 



We must now proceed to test these hypotheses by physiological ex- 

 periment, for, if they are found to apply to the intracranial circulation, 

 the conclusion becomes inevitable that changes in the total blood supply 

 to this, the most important organ in the body, are dependent not on any 

 local adjusting mechanism in that organ itself, but upon conditions pre- 

 vailing in other parts of the body, with the possibility that a local vaso- 

 dilatation may be provided for by a secondary compression of neigh- 

 boring venules, or perhaps even by an active constriction of the arteri- 

 oles of neighboring inactive centers. 



The questions of greatest practical importance are, therefore, as fol- 

 lows: (1) What determines the intracranial pressure, and how does this 

 vary during each heart beat? (2) If there can be no change in the actual 

 volume of blood in the vessels as a whole, what provision is made to 

 cause changes in blood supply with varying degrees of activity of the 

 brain, and how are these changes brought about? (3) Is it possible with- 

 out change in the total volume of blood in the brain for certain vascular 

 areas to expand at the expense of others that correspondingly constrict? 



The Pulsations of the Brain and the Cause of Intracranial Pressure. 

 Examination either of the fontanelles in an infant or of the surface of 

 the brain exposed by trephining shows distinct pulsations, but this does 

 not prove that similar pulsations occur in the intact brain, for the ab- 

 se.nce of a part of the cranial wall might be responsible for the pulsa- 

 tion. The presence or absence of pulsation must be sought for in the 

 still rigid brain case. This has been done by closing a trephine hole by 

 a glass window through which the cranial contents can be seen when 

 strong illumination is used : pulsations of the vessels are clearly visible. 

 To determine the exact relationships of the pulsations, the trephine hole 

 is connected with a delicate recording tambour by screwing into it a 

 brass tube closed at its inner end by a thin rubber membrane. It has 

 been found that the arteries expand somewhat with each cardiac sys- 

 tole, and that there are further expansions with each expiration, but not 

 with inspiration, as is the case in other vascular areas. The room for 

 the expansions is no doubt provided mainly by compression of the 

 cerebral veins, thus causing the blood within them to exhibit corres- 

 ponding waves of pressure. The reason why expiration and not in- 

 spiration causes the increase in volume is that there are no efficient valves 

 between the right side of the heart and the cerebral veins. This allows 



