504 ANIMAL HEAT 



the temperature of the surrounding air exceeds that of the blood. We must 

 remember, too, that all food and drink which enter the body at a lower tem- 

 perature abstract a small measure of heat; while the urine and feces which 

 leave the body at about its own temperature are also means by which a certain 

 small amount of heat is lost. 



Heat Lost from the Surface of the Body. By far the most impor- 

 tant loss of heat from the body, probably 90 per cent, and upward of the 

 whole amount, is that which takes place by radiation, conduction, and the 

 evaporation of moisture from the skin. The actual figures for ordinary 

 conditions are as follows: For every 100 calories of heat produced, 2.6 are 

 lost in heating the food and drink; 2.6 in heating the air inspired; 14.7 in 

 evaporation; and 80. i by radiation and conduction. During increased 

 activity of the body the proportion of heat lost by evaporation is greatly 

 increased. The means by which the skin is able to act as one of the most 

 important organs for regulating the temperature of the blood are, i, that 

 it offers a large surface for radiation, conduction, and evaporation; 2, that it 

 contains a large but adjustable amount of blood, and the quantity of blood 

 is greater under those circumstances which demand a loss of heat from the 

 body, and vice versa, which gives a means for varying the loss of heat by 

 radiation and conduction; 3, that it contains the sweat glands, which dis- 

 charge a quantity of moisture to be evaporated from its surface. 



The circumstance which directly determines the quantity of blood in the 

 skin is that which governs the supply of blood to all the tissues and organs 

 of the body, namely, the power of the vaso-motor nerves to cause a greater 

 or less tension of the muscular element in the walls of the arteries, and, in 

 correspondence with this, a lessening or increase of the caliber of the vessel, 

 accompanied by a less or greater current of blood. A warm or hot atmos- 

 phere so acts on the sensory nerves of the skin as to lead to a reflex relaxa- 

 tion of the muscular fiber of the blood vessels; as a result, the skin becomes 

 full-blooded, relatively hot, and moist from sweating; and much heat is lost. 

 With a low temperature the blood vessels shrink, and with the consequently 

 diminished blood supply, the skin becomes pale, cold, and dry, an effect 

 produced through the vascular centers in the medulla and spinal cord. 



The activity of the sweat glands of the skin is also regulated reflexly 

 through the sweat centers. The increased blood supply just described is 

 favorable to increased production of sweat by the sweat glands. Thus, 

 by means of the self-regulation the skin becomes the most important of the 

 means by which the temperature of the body is regulated. 



The relative loss of heat by the means given, i.e., radiation, conduction, 

 and evaporation, will depend on two factors: first, the relative temperature 

 of the body to the surrounding air; and, second, the humidity of the air. If 

 the atmospheric temperature is the same as that of the body, of course there 

 will be no loss of heat by radiation and convection; if the air temperature is 



