ANIMAL HEAT 447 



(when engaged in active muscle movements produced 271.2 calories and 

 bsorbed 119.84 grams of oxygen per hour. The increase in heat-produc- 

 tion per hour during activity was thus almost doubled, though the sum total 

 produced daily in which there was a working period of eight or ten hours 

 was only about one-third more than during a day of repose. During sleep 

 here is a greatly diminished heat-production, not more than 40 calories per 

 hour being produced. The preceding data may be tabulated as follows 

 Him) : 



Day of Rest. Day of Work. 



Heat units (Calories) pro- } Rest 16 hrs. Sleep 8 hrs. Rest 8 hrs. Work 8 hrs. Sleep 8 hrs. 

 duced / 2470.4 320 1235.2 2169.6 320 



2790.4 3724-8 



Increased muscle activity is associated with increased metabolism and 

 k more rapid dissipation of heat, which must be compensated for by a cor- 

 responding heat-production if heat equilibrium is to be maintained. This 

 necessitates a larger amount of food ingested. From Calorimetric inves- 

 tigations, as well as from a calculation based on the amounts of the excreted 

 products, various diet scales have been arranged by different investigators, 

 which it is believed will furnish the extra amount of energy. A few are 

 here appended. 



DIETARIES FOR A MAN OF 70 KILOS, DURING HARD WORK 

 Voit. Rubner. Playfair. Hultgren. 



Protein 145 165 155 134 grams 



Fat 100 70 70 79 grams 



Carbohydrate 500 565 567 522 grams 



3574 33 62 3 6l 9 343 6 Calories 



The Mechanism of Heat-production. Heat is liberated to a greater 

 or less degree in all tissues in the cells of which food materials^ are under- 

 going disruption and oxidation or metabolism. The tissues in which metabo- 

 lism and therefore heat liberation takes place most energetically, e.g., muscles 

 and glands are more especially to be regarded as thermogenic tissues. ^ 



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 com- 

 ing through afferent nerves from the muscles themselves, the joints, tendons, 

 and skin. As a result of this slight but constant stimulation of the spinal 

 cord, the metabolic changes in muscle material are maintained 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-produc- 

 tion. Therefore work and exercise of all kinds which involve a more rapid 

 contraction of the skeletal muscles is attended with increased heat-produc- 

 tion. By reason of their mass and more or less continuous activity the muscles 

 are justly regarded as the chief thermogenic organs. 



