430 MANUAL OF PHYSIOLOGY. 



value, i.e., the number of heat-units its combustion will produce, 

 we ought to be able to estimate the amount of heat produced by 

 ascertaining this value and subtracting the calorific value of the 

 various substances given off by the body. Since practically the 

 temperature of the body remains the same, the amount of heat lost 

 during a given time should correspond to the income, estimated 

 from the number of heat-units of the food. So far, however, at- 

 tempts to make the calculated heat-income correspond with the 

 expenditure have not been productive of satisfactory results, the 

 estimated calorific value of the food being hardly equal to the 

 heat, calculated to be given off by the body. We must remem- 

 ber that it is not the proteid, fat and starch of the body, that we 

 burn, but the living tissues formed by the assimilation of these 

 substances. We do not know what chemical changes go on in 

 the steps of tissue formation, and therefore we cannot say exactly 

 what combinations are submitted to the combustion which gives 

 us a high heat- value. 



Since the activity of muscle and gland tissue is constantly un- 

 dergoing variations in intensity, the amount of chemical change 

 differs at different times, so that the amount of heat produced must 

 also vary. We know that the heat set free by any organ, such 

 as a gland or a muscle, increases in proportion to the increase of 

 its functional activity, but we cannot say that the calorific activity 

 can vary independently of other circumstances. Without such 

 a special calorific function of some tissues, such as muscle, the 

 actual net heat-income must vary with circumstances which are 

 accidental and therefore irregular. 



Since we know that the nervous system controls the tissue ac- 

 tivities which are accompanied by the setting free of heat, we can 

 see how the nerve centres can materially influence the heat pro- 

 duction of the body ; thus, the more active are the muscles, 

 glands, etc., which are under the control of nerves, the greater 

 the amount of heat produced in a given time. That the nervous 

 system can cause in any tissue a chemical change, giving rise to 

 a greater production of heat, without any other display of func- 

 tional activity, we do not know, but many facts seem to point to 

 such a possibility. 



