45 2 TEXT-BOOK OF PHYSIOLOGY 



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. 



The consumption of foods that have a high 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 

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

 will be an increase in heat-production. 



The chief external factor that increases metabolism in muscles and 

 other organs and tissues is a low external temperature. With a given 

 external temperature such as characterizes the fall months the metabolism 

 and heat production attain a certain level. As the temperature falls 

 duting the winter months there will be an increased stimulation of cuta- 

 neous nerves, increased stimulation of spinal cord centers, and in conse- 

 quence increased activity of muscles, glands and other tissues as well. 

 The increased metabolism thus developed leads to the consumption of 

 increased amounts 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 temperature coexist, heat-production attains its maximum, amount- 

 ing to as much as 4726 Calories daily (Hultgren). 



HEAT-DISSIPATION. THERMOLYSIS 



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 



