404 ANIMAL HEAT AND ITS REGULATION 



It is therefore impossible to give definite figures for the amount of water 

 given off through the skin; for all of the twenty-four-hour determinations 

 thus far published have reference only to the total output of water vapor, 

 and not to the apportionment between lungs and skin. 



Using the above-mentioned figures (table, page 403) for the output of water 

 vapor in respiration, we can form some approximate idea of the amount of 

 water vapor eliminated by the skin. Some examples are adduced here in which 

 the necessary reductions have already been made. A fasting individual. gave off 

 through the skin on the fifth day of his fast approximately 350 g. of water ; loss 

 of heat 188 Cal. The same individual on a plentiful diet gave off through the 

 skin 710 g. water, loss of heat 381 Cal., twice as much therefore as in the first 

 case. Another subject at rest and on a moderate diet excreted through the skin 

 480 g. of water ( = 258 Cal.) ; on the same diet at severe work, 1,280 g. water 

 (= 686 Cal.) (cf. also page 397). 



According to Zuntz, soldiers on the march in cold weather eliminate about 

 one-fifth of the total output of water through the respiration, in warm weather 

 only about one-sixth. In extreme cases the loss of heat by evaporation may 

 reach the enormous value of ninety-five per cent of the total heat loss. 



The loss of heat by radiation and conduction also exhibits great variations, 

 which depend on the temperature of the surrounding air and the heat produc- 

 tion going on in the body, as well as upon the clothing. Both radiation and 

 conduction are considerably greater on exposed parts of the skin than on clothed 

 parts. From experiments on covered portions, the total heat loss by radiation 

 has been calculated for a grown man at about 700-800 Cal., while from obser- 

 vations on the naked skin, it is estimated at 1,700-1,800 Cal. Similar differences 

 have been observed with regard to loss of heat by conduction. 



4. PROTECTION AGAINST LOSS OF HEAT 



Notwithstanding considerable variations in the temperature of the sur- 

 rounding air, homoiothermous animals maintain an even balance between heat 

 production and heat loss. That this is possible with so small a quantity of 

 heat production in the body is due to the fact that provisions are made in 

 warm-blooded animals for restraining the loss of heat through the skin. These 

 provisions are, (1) the subcutaneous adipose tissue, and (2) the natural hairy 

 or feathery covering of the body. 



We have already seen that the muscles come first in the order of heat pro- 

 duction. The heat formed in them, however, cannot be readily conducted to the 

 super jacent skin because the intervening adipose tissue is a very poor conductor 

 of heat. A piece of skin 2 mm. in thickness will allow 0.00248 cal. (small) to 

 pass through in one minute, when the difference in temperature is 18.2 C. The 

 same piece of skin plus a 2 mm. layer of fat under the same circumstances will 

 allow only 0.00123 cal. to pass. With less difference in temperature on the two 

 sides the protecting influence of the fat is still greater, so that for example with 

 a difference of 9 C. a layer of fat 2 mm. in thickness retains 0.8 of the total 

 quantity of heat which would otherwise be allowed by the skin to pass through 

 (Klug). In considering the properties of fat deposited at different places in 

 the body, Henriques and Hansen have directed attention to the lower melting 

 point of fat which lies nearer to the outer surface of the body. This is doubt- 





