736 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



that the beneficial effects which in so many instances follow trephining of the 

 skull 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 x has found the tempera- 

 ture of the blood coming from the brain (dogs) 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 suffi- 

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



As 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 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 of Oxygen and Carbonic Acid in various Samples of Dogs' 



Blood (Hill). 2 

 Average of 52 arterial samples | CO ' ^7. 64 percent. 



Average of 42 torcular samples in 2 



Average of 28 femoral vein { ^ 2 



The absolute quantity of 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 be remembered, however, that 

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

 less than one-tenth of the entire encephalic mass. 



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

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

 blood is withdrawn from the limbs ; the blood thus withdrawn can be shown 

 to pass toward the trunk, for when a person lying on a horizontal table sup- 

 ported at the centre on a transverse knife-edge is just balanced, then increased 

 activity of the cerebral centres causes the head end to dip down (Mosso), and 

 if the skull wall is defective the brain is seen to swell. 



In the latter 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 of the cerebral circulation in the case of those in whom the 

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

 ing into the cranial cavity as a result of mental effort or emotion. In the 



1 Die Temperatur des Gehirns, 1894, Leipzig. 2 Journal of Physiology, vol. xviii., 1895. 



