208 THE HUMAN MECHANISM 



intestine. This cannot readily be secured when blood is 

 being sent in large quantities to the skin in order to cool 

 the body. We have seen all along that the two great vas- 

 cular areas of the skin and digestive organs are more or less 

 antagonistic or compensating in their vasomotor reactions. 

 When the blood is present in large quantities in the skin, 

 it is present in smaller quantities in the stomach, the intes- 

 tine, the pancreas, the liver; and, vice versa, these organs 

 can best obtain an adequate blood supply when the, de- 

 mands of the skin are not excessive. Consequently diges- 

 tion is more difficult in warm than in cold weather, and we 

 should then eat less at a time, even if we have to eat 

 somewhat more frequently. 



During the digestion of a meal the chemical activities of 

 secretion, the peristaltic muscular movements, etc., somewhat 

 increase heat production in the body; and this increase, 

 though not great, is at times great enough to make us feel 

 distinctly warmer. When one is slightly chilly, for example, 

 he often feels warmer after eating something, even though 

 the meal be cold; and on a very warm, muggy day, when 

 the blood flow through the skin is already excessive and its 

 temperature unduly high, the digestion of a meal often adds 

 to the discomfort, because the larger production of heat leads 

 to further dilation of the skin vessels. 



23. The mechanism of temperature regulation. The pre- 

 ceding pages have shown us that temperature regulation 

 depends chiefly on three physiological mechanisms: (1) the 

 vasomotor system, which controls the distribution of blood 

 between the skin and the internal organs ; (2) the sweat 

 glands; (3) the mechanism of heat production. The first 

 of these has already been described in the study of the 

 circulation. The heating of the skin stimulates afferent 

 nerves which reflexly dilate the arteries of the skin and 

 also simultaneously constrict those of internal organs. This 

 reflex, then, is dependent on the temperature of the skin ; 



