64 THE APPLIED PHYSIOLOGY 



THE INTRACRANIAL CIRCULATION. 



There are two important regions of the body in 

 which the ordinary laws regulating the blood-supply 

 do not hold good, and to the consideration of these 

 we now turn. We will speak first of the intracranial 

 circulation. 



It is an obvious fact that one cannot put more 

 than a pint into a pint pot, and since the brain is 

 incompressible, the total volume of blood inside the 

 bony cranial cavity cannot vary. It is true that the 

 volume of the cerebrospinal fluid is not absolutely 

 constant. A trifling ebb and flow is possible along 

 the sheaths of the spinal nerves. The occipito- 

 atlantal membrane pulsates a little, and so does the 

 membranous fontanelle in an infant. Nevertheless, 

 the total volume of this fluid is so small that to 

 all intents and purposes it does not alter the truth 

 of the fact propounded. 



Yet every organ, and surely the brain amongst 

 the rest, needs more blood during moments of 

 activity or excitement, and less during rest or sleep. 

 At least, the supply of oxygen and nutriment must 

 roughly correspond to the demand. How may this 

 be brought about ? 



Although the volume of blood in the cranial cavity 

 at any given moment must be a constant, there are 

 two ways in which more oxygen can be supplied. In 

 the first place, the arteries might be dilated at the 

 expense of the veins. In the second place, the rate 

 of flow might be increased, so that in a given space of 

 time a larger volume should have passed through the 



