250 THE CIRCULATION OF THE BLOOD 



to see how such reciprocal dilatation and constriction could be of any 

 advantage except perhaps in causing certain areas to receive more 

 blood than others. A reciprocal relationship might also exist between 

 adjacent arterioles as well as between arterioles and veins ; when, for 

 example, the arm center becomes active, it is conceivable that its 

 arterioles might dilate at the same moment that those of a neighboring, 

 less active center become constricted. Alterations obviously might oc- 

 cur without causing any perceptible change either in the volume of the 

 brain as a whole or in the condition of venous flow. 



In consideration of these factors, most observers are agreed that the 

 total volume of blood in the brain must be constant at all times (Monro 

 and Kellie doctrine). Alteration of blood supply can, however, still be 

 brought about by changes in the velocity with which the blood traverses 

 the vessels. When more blood is required in the brain to supply the 

 increased metabolism which we must presume accompanies heightened 

 mental activity, it is not accomplished as in other parts of the body by 

 an increase in the capacity of the vessels as compared with those of 

 other vascular areas, but by a hurrying up of the circulation through 

 vessels whose caliber remains unaltered. 



The main factors determining the velocity of bloodflow through the 

 brain must, therefore, be dependent upon changes occurring elsewhere 

 in the vascular system, a conclusion for which there is abundant experi- 

 mental evidence. Of the many ingenious methods that have been de- 

 vised to secure this evidence, we will cite but one in this place. Records 

 are taken of changes in: (1) the venous Hood pressure of the brain by 

 connecting a cannula either with the vein immediately after leaving the 

 skull gr, better still, with the torcular Herophili; (2) the brain volume, 

 by connecting a very sensitive receiving tambour with a trephine hole 

 in the cranium so that its open end lies against the pia mater.* Al- 

 though, as we have seen, while incased in the rigid cranium the brain 

 volume can not change to any degree, yet this will occur when a 

 portion of the cranium is removed, so that pulsations correspond- 

 ing to those in the blood vessels will be observed; (3) the circula- 

 tory conditions elsewhere in the body, by taking arterial and 

 venous pressures and plethysmograms. The results in a normal an- 

 imal show the following points (see Fig. 80): (1) The tracings of 

 the arterial blood pressure (A), the brain volume (C) and the intra- 

 cranial venous pressure (C) have exactly the same contour that is, 

 the respiratory and the cardiac waves in all three of them are identical. 

 The venous blood as it flows into the jugular veins also pulsates in 



*This receiving tambour really consists of a brass tube of the same diameter as the trephine 

 hole, into which it is tightly fitted. The brass tube is closed at its inner end by thjn rubber membrane, 

 and its outer end is connected with the receiving tambour. 



