448 TEXT-BOOK OF PHYSIOLOGY 



The consumption of foods that have a higher potential heat value also 

 contribute to the amount of heat produced. Foods have different physio- 

 logic heat values. If the food consumed contains much potential energy and 

 quantity consumed be larger than the average daily requirements, there will 

 be an increase in heat-production. (See page 442.) 



The chief external factor that increases metabolism in these and other 

 organs and tissues is a low external temperature. A fall of the external 

 temperature, such as is experienced in the fall and winter seasons, causes, 

 through cutaneous afferent nerve stimulation, a stimulation of the spinal 

 motor centers, and to a larger discharge of nerve impulses to muscles, glands, 

 and other tissues. The. increased metabolism thus developed leads to the 

 consumption of increased amount of food and oxygen. As a result there is 

 an increased heat-production. If at the same time volitional activities of 

 muscles be evoked, as is not infrequently the case, there will be a still further 

 increase in metabolism and heat liberation. When all these conditions, 

 increased muscle activity, increased amount of food with high potential 

 energy, and a low external temperature coexist, heat-production attains its 

 maximum, amounting to as much as 4726 Calories daily (Hultgren). 



HEAT-DISSIPATION. THERMOLYSIS 



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From the preceding statements it is evident that the body is continually 

 liberating heat in amounts daily, far in excess of that necessary for the main- 

 tenance of the body-temperature. Should this heat be retained, the tem- 

 perature of the body would be raised at the end of twenty-four hours, an 

 additional 18 or 2oC. a temperature far in excess of that compatible 

 with the maintenance of physiologic processes. That the body may be 

 kept at the mean temperature of 37C. it is essential that the heat liberated 

 be dissipated as fast as it is produced, or to state the problem in another 

 way, the heat dissipated by the body must be replaced by an equal amount 

 liberated, if equilibrium of temperature is to be maintained. The dis- 

 sipation of the heat is accomplished in several ways: (i) In warming the 

 food and drink to the temperature of the body. (2) In warming the inspired 

 air to the same temperature. (3) In the evaporation of water from the 

 lungs. (4) In evaporating water from the skin. (5) In radiation and con- 

 duction from the skin. The quantities of heat lost to the body by these 

 different processes it is difficult for obvious reasons to accurately determine, 

 and the estimates usually given must be regarded only as approximative. 



The number of Calories which are theoretically liberated by the various 

 diet scales can be readily determined by multiplying the quantities of food 

 principles consumed by the usual factors (see page 442). Thus the total 

 number of Calories furnished by the various diet scales would be as follows: 

 Voit's, 3387; Vierordt's, 2695; Ranke's, 2335; Moleschott's, 2984; Atwater's, 

 3331; Hultgren's, 3436. As the body- weight may not increase and as the 

 temperature in physiological conditions does not rise, the assumption is 

 that the food is oxidized in the body to urea, carbon dioxid and water with 

 the liberation of the foregoing amounts of heat and their equally rapid 

 dissipation. 



Assuming 2500 Calories to be an average of heat liberated during a day 

 of repose, the losses, in the ways stated in the foregoing paragraph, may be 

 tabulated as follows: 



