THE CIRCULATION OF THE BLOOD 157 



to the level of the general surface of the head when the 

 subject awakens. 



Upon this point of diminished blood supply to the brain 

 during sleep almost all physiologists are agreed; there is also 

 general agreement that the arm and the leg increase in vol- 

 ume when we go to sleep, and this is thought to be due to 

 a dilation of the arteries of the skin. It is very significant, 

 on the other hand, that the arm shrinks in volume when the 

 brain is active in mental work, and especially in mental work 

 involving the personal interest or mental concentration of the 

 subject of the experiment. 



It is thought by some that other vascular areas that of 

 the abdominal cavity, for example behave in this respect in 

 the same way as the skin, but on this point the evidence is 

 not conclusive. It is, indeed, not improbable that these other 

 vascular areas play some part in the regulation of the flow 

 to the brain, but it is not likely that they stand in the same 

 intimate relation to it as does the skin. 



The fact is clear, however, that a close relation exists 

 between cutaneous circulation and the maintenance of proper 

 vascular conditions in the brain. Mental work, for example, 

 is more difficult for most people in very warm weather 

 because at that time the cutaneous arterioles are widely 

 dilated ; and, on the other hand, it is easy to understand 

 why the constriction of the vessels of the skin by cold makes 

 it difficult to go to sleep without sufficient bedclothing. 



19. The circulation during the digestion of a meal. After 

 eating a meal more blood is needed in the secreting diges- 

 tive glands (especially the stomach and pancreas) and also 

 in the intestinal organs of absorption, the villi. This need 

 is greatest during the first hour or two, when there is the 

 largest amount of food to be worked upon. We find, accord- 

 ingly, that the arteries of these organs then dilate to such 

 an extent that the mucous membrane of the stomach and 

 intestine, which is pale pink while those organs are at rest, 



