HEAT INCOME. 433 



matter assimilated. As each kind of food has a certain heat 

 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 

 excreta and the energy used in producing the muscular move- 

 ments of 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, attempts 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 sufficient to produce the heat cal- 

 culated to be given off from and the other work done by the 

 body. We must remember 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 in- 

 crease of its functional activity, but we cannot say that the calo- 

 rific 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 

 activities which are accompanied by the setting free of heat, we 

 can see how the nerve centres can materially influence the heat 

 production 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- 

 37 



