ANIMAL HEAT. 435 



products of the foods and drink from the body an equal amount of 

 heat is carried out. 



2. In Warming the Inspired Air. The average temperature of the air 



is 12 C.; the amount of inspired air, about 15 kilograms; the specific 

 heat of air, 0.26. The absorption of body-heat by the air until it at- 

 tains the temperature of the body will therefore amount to 15X0.26X25 

 = 97.5 Calories = 3. 8 per cent. The expired air removes from the 

 body a corresponding amount. 



3. In the Evaporation of Water from the Lungs. The quantity of water 



evaporated from the lungs may be estimated at 400 grams; as each 

 gram requires for its evaporation 0.582 Calorie, the quantity of heat 

 lost by this channel would be 400X0.582 = 232.8 Calories=9.4 per 

 cent. 



4. In the Evaporation of Water from the Skin. The quantity of water evapor- 



ated from the skin may be estimated at 660 grams, causing a loss of 

 heat by this channel of 660X0.582 = 384.1 Calories=i5.3 per cent. 



5. In Radiation and Conduction from the Skin. The amount of heat lost 



by this process can be indirectly determined only by subtracting the 

 total amount lost by the above-mentioned channels from the total 

 amount produced. Thus, 2500777.44= 1725.6 Calories = 69 per cent, 

 would represent the average amount lost by radiation and conduction. 

 Regulation of the Mean Temperature. In order that the mean 

 temperature of the body may remain practically constant, the heat dissipated 

 must be exactly balanced by the heat liberated. Should there be any want 

 of correspondence between the two processes, there would arise either an 

 increase or a decrease in the mean temperature. As both heat-production 

 and heat-dissipation are variable factors, dependent on a variety of internal 

 and external conditions, their adjustment is accomplished by a complex self- 

 regulating mechanism involving muscle, vascular, and secretor elements, 

 coordinated by the nerve system. 



Heat-Production. Heat-production varies in intensity and amount, 

 in accordance with a number of conditions, but principally with variations 

 in physiologic activity, the quantity and quality of the food, and changes in 

 the external temperature. It will be recalled that all muscles possess tonicity 

 by which is meant a slight degree of contraction, the result of the continuous 

 arrival of nerve impulses through efferent nerves discharged from motor 

 nerve-cells in the spinal cord this discharge being maintained largely by 

 nerve impulses coming through afferent nerves from the muscles themselves, 

 the joints, tendons, and skin. As a result of this slight but constant stimula- 

 tion of the spinal cord, the metabolic changes in muscle material are main- 

 tained at a certain level, with a corresponding liberation of heat. The chief 

 result of the tonicity would thus be the production of heat. Any physiologic 

 condition that leads to a greater discharge of nerve impulses from the spinal 

 cord and hence increased muscle activity, must be attended by increased 

 heat production. Therefore work and exercise of all kinds which involve 

 a more rapid contraction of the skeletal muscles is attended with increased 

 heat production. The consumption of foods that have a higher potential 

 heat value also contribute to the amount of heat produced. Foods have 

 different physiologic heat values. If the food consumecl contains much 



