ANIMAL HEAT. 343 



radiation. The entire body is covered by a corneous envelope 

 formed by the superficial layers of the epidermis. The greater 

 part of the body is, moreover, covered with down or hair, en- 

 closing a layer of air, which forms a covering as little adapted 

 to be a conductor of heat as the layers of the epidermis. 

 Finally, a special layer of areolar tissue is found in the 

 dermis (see for all these parts, Physiology of the external 

 Integument), called the subcutaneous tissue, or adipose 

 pannicle, formed of. cells filled with fat, and affording a pro- 

 tecting envelope as regards heat, and appearing most highly 

 developed in cases in which the loss of heat appears most 

 probable (as in that of newly born animals and animals of the 

 polar regions). We also possess numerous and important 

 blood currents, circulating with much greater activity than is 

 required for the purpose of nutrition, in the parts more par- 

 ticularly exposed to cold, such as the pinna of the ear, the 

 face (especially the nose), the hands, and the extremity of 

 the fingers; these currents considerably increase the heat of 

 these parts of the body. 



The organism has more difficulty in withstanding a too 

 great rise in the external temperature. For this purpose we 

 find that organs are employed which we have mentioned as 

 being endued with very slight conducting power, such as the 

 cells of the epidermis, the air enclosed in the pilous covering 

 or hair, and the adipose panicle. The most effectual means, 

 however, of resisting too great an elevation of temperature 

 are found in the phenomena of evaporation which take place 

 in the lungs and on the surface of the skin. 



With regard to the lungs, we know that, in general, while 

 the 10 cubic metres of air inhaled in 24 hours contain only 

 from 50 to 60 grms. of vapor, the air exhaled contains, on an 

 average, from BOO to 400 grms., and often more : we find, by 

 calculation, that from 200 to 300 heat-units are probably em 

 ployed in turning this water into vapor at 35 or 36 (C.) 

 (the temperature of the exhaled air) ; this loss of Leat may 

 be carried to a much higher point ; for instance, in animals 

 which, like the dog, scarcely perspire at all, except through 

 the lungs, it forms the principal means of equilibrium of the 

 internal heat, when this is increased to too great a degree, as 

 in violent exercise, running, etc. 



The evaporation of sweat, from the surface of the skin, is 

 the principal means possessed by man of withstanding an 

 excess of heat. We shall consider this subject at greater 

 length when studying the functions of the skin, especially the 



