PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 257 



is also possible along the sheaths of certain of the cranial nerves. Re- 

 cent investigation has, however, clearly demonstrated that under nor- 

 mal conditions the amount of cerebrospinal fluid is too limited to make 

 it of any significance in this connection. As explained on page 255 the 

 main function of the fluid is to equalize the pressure between the brain 

 and the cord. 



Although it is therefore improbable that the vessels as a whole could 

 expand or contract, it is still possible that some provision might exist 

 by which extra room could be made to allow of localized dilatation of 

 certain parts of the vessels. The veins, for example, might contract 

 in proportion as the arteries dilated and the possibility becomes all the 

 more likely when we consider that because of the great capacity of the 

 cerebral veins, their lumina might be considerably constricted without 

 any serious obstruction being offered to the bloodflow through them. 

 Such a reciprocal dilatation and constriction of the proximal and dis- 

 tal halves of a thin-walled rubber tube suspended in water in a closed 

 flask can be demonstrated provided some resistance be inserted between 

 the two halves. This resistance would be represented in the intracra- 

 nial vessels by the capillary area. It is impossible to say to what extent 

 this reciprocal mechanism between arteries and veins may prevail, but 

 in any case it can not well extend beyond the cerebral veins to the 

 sinuses, since these are partly embedded in the cranium itself and are 

 protected by relatively thick membranes on their free sides. The mech- 

 anism may be employed for permitting the arteries of a local area to 

 expand, but it can not obtain over any large area, since otherwise the 

 total outflow of blood from the sinuses through the jugular foramen 

 would be curtailed, which we know to be contrary to what actually oc- 

 curs when the arterial pressure is raised, and which moreover would be 

 highly detrimental, since it would cause self-strangulation of the intra- 

 cranial bloodflow. 



These physical considerations lead us to expect that there can not be 

 any dilatation or constriction in the intracranial vessels which is com- 

 parable with that which occurs in other vascular areas, although it may 

 take place to a degree which is limited by the extent to which the cere- 

 bral veins can be passively contracted or expanded without curtailment 

 of the bloodflow. Acting to this extent, the dilatation produced in the 

 arteries by each cardiac systole accounts for the rise in pressure which 

 occurs simultaneously in the venous sinuses (as measured in the torcu- 

 lar Herophili), but it is unlikely that the amount of blood supplying the 

 brain will be determined by local dilatation or constriction of the blood 

 vessels, as is the case, for example, in a gland or muscle. Of this we 

 are certain, that the total volume of blood within the brain case at any 



