SEC. 6. THE CIRCULATION IN THE BRAIN. 



The supply of blood to the brain seems at first sight not to 

 correspond to the importance of this the chief organ of the body. 

 In the rabbit it would appear that not much more than one per 

 cent, of the total quantity of the blood of the body is present at 

 any one time in the brain, a quantity distinctly less than that 

 which is found in the kidneys; and of the total weight of the 

 organ, the weight of blood in the brain at any one time amounts to 

 about five per cent., being about the same as in the muscles, whereas 

 in the kidney it amounts to nearly twelve per cent, and in the liver 

 to as much as nearly thirty per cent. Making every allowance for 

 the relative small size and functional importance of the rabbit's brain, 

 the blood-supply of even the human brain must still be small. In 

 other words, the metabolism of the brain-substance is of importance, 

 not so much on account of its quantity as of its special qualities. 



We have seen (p. 366) in speaking of respiration that when 

 the brain is exposed the quantity of blood in the brain and so the 

 total volume of the brain rises and falls, in a conspicuous manner, 

 with the respiratory movements. And observations by the plethys- 

 mographic method, a portion of the skull being removed for the 

 purpose or advantage being taken of a natural deficiency, have shewn 

 the existence of more rapidly repeated movements, of a swelling 

 and shrinking synchronous with and due to the beats of the heart, 

 as well as of variations, larger and slower than the respiratory 

 undulations, and brought about by various causes such as the 

 position of the head in relation to the trunk, movements of the 

 limbs, modifications of the respiratory movements, and apparently 

 phases of activity of the brain itself, as in waking and sleeping 



