288 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the >kull may depend upon this mechanical release. Of course, in cases with 



a defective skull-wall an increase in arterial pressure causes a more decided 

 increase in the volume of blood in the brain ; this, however, is much more 

 marked than it would be under ordinary conditions, and is not to be regarded 

 as the main effect, which is an increase in the quantity of the blood passed 

 through the central system in a unit of time. Mosso ' has found the tempera- 

 ture of the blood coming from the brain (dog's) slightly higher than that of 

 the rectum and of the arterial blood. The differences are very small, but he 

 draws the conclusion that the metabolic processes in the brain are sufficiently 

 intense to raise the temperature of the blood passing through it. 



A.S against the intensity of the metabolism in the central system, it has 

 been observed that blood taken from the torcular Herophili of the dog was 

 Intermediate in gaseous content between arterial blood and that taken from 

 the femoral vein, thus indicating that the arterial exchange was less intense 

 in the brain than in the muscles of the leg. The following is a condensed 

 statement of the figures : 



Percentages <>/ Oxygen and Carbonic Acid in virions Samples of Dogs' Blood 



, c ... , . , . i <<>.,. . 37.64 percent 



Average oi 52 arterial samples lO 18 25 " 



a r aoi i i fC0 2 . . 41.65 " 



Average of 42 torcular samples lO 13 49 " 



Average of 28 femoral vein samples - -' „'L. 



The absolute quantity of the blood in the brain and cord is certainly 

 small ; if we may judge from the observations on animals, it is not more 

 than 1 per cent, of the entire blood in the body. It is to remembered, how- 

 ever, that the cell-bodies, which alone are well supplied with blood, probably 

 represent less than 2 per cent, of the entire encephalic mass. 



With general rise and fall of pressure elsewhere, there is a rise and a fall 

 of pressure within the central system. During the first phases of mental 

 activitv blood is withdrawn from the limbs ; the blood thus withdrawn can 

 be shown to pass toward the trunk' and head, for when a person lying on a 

 horizontal table supported at the centre on a transverse knife-edge is just 

 balanced, then increased activity of the cerebral centres causes the head to 

 dip down (Mosso), and if the skull-wall is defective the brain is seen to swell. 



In the later stages of fatigue the blood-supply to the nerve-centres dimin- 

 ishes owing to a decrease in force of the heart-beat and the tonicity of the 

 splanchnic vessels, SO that the brain in birds exhausted by a long flight 

 has been found by Mosso to be in a high degree anaemic. There is much 

 reason to think that in man a similar reaction occurs. 



The study <>f the cerebral circulation in the ease of those in whom the 

 skull-wall is at sonic point deficient shows a bulging of the skin over the 



1 I>i> Temperatur des Gehirns, Leipzig, 1894. 



2 Journal of Physiology, 1895, vol. xviii. 



