436 MANUAL OF PHYSIOLOGY. 



greatly increased ; while, at the same time, so much blood being 

 occupied in circulating through the skin, the deeper heat-pro- 

 ducing tissues are supplied with less blood, and therefore gener- 

 ate a lesser quantity of heat. Thus a marked rise in the external 

 temperature, which at first sight would seem to impede the escape 

 of heat from the body, really facilitates it, by causing, through 

 the vascular and glandular nerve mechanisms of the skin, a greater 

 exposure of the blood to the cooler air, and a greater quantity of 

 moisture to be evaporated from the warm skin. When the tem- 

 perature of the air reaches that of the body, then the only way 

 of disposing of the heat generated in the body is by evaporation, 

 for radiation and conduction become impossible. In animals like 

 man, whose cutaneous moisture is so great, external heat seldom 

 causes marked change in the rate of breathing, but in animals 

 whose cutaneous secretion is limited, external heat distinctly affects 

 their respiratory movements, as may be seen by the panting of a 

 dog on a very warm day, even when the animal is at rest. 



Almost more important than facilitating the escape of heat in 

 very warm weather, are the arrangements for preventing its loss 

 when the surroundings are unusually cold. In this case the cold 

 acting as a stimulus to the vaso-constrictor nerve agencies of the 

 skin causes the blood to retire from the surface and fill the deeper 

 organs, where more heat is produced. This bloodless skin and 

 the underlying fat then act as a non-conducting layer or boundary 

 protecting the warm blood from the cooling exposure. At the 

 same time the secretion of the sweat is checked by a special nerve 

 mechanism. Here, too, the cold air, which would soon rob the 

 moist surface of its caloric, checks the secretion and thereby nul- 

 lifies its effects in this direction, and enables the body to remain 

 at the normal standard temperature. 



The chief factors that regulate the body temperature belong 

 then to the expenditure department, and may be said to be (a) 

 variation in the quantity of blood exposed to be cooled, and (6) 

 variation in the quantity of moisture exposed for evaporation. 

 These regulators have to compensate not only for differences of 

 external temperature, but also for great fluctuations in the amount 

 of heat produced in the tissues. 



The regulating power of the skin, etc., appears to be adequate 



