HEAT-INCOME. 429 



this correspondence remains whether the dissociation takes place 

 rapidly or slowly. The substances we make use of as food have 

 thus a certain heat value which depends upon their chemical 

 composition. 



The high temperature which homoeothermic animals can keep 

 up in spite of the cold of the atmosphere in which they live is 

 readily accounted for by the chemical change which is constantly 

 occurring in the tissue of their bodies. 



The amount of heat produced in any part must, then, depend 

 upon the activity of its tissue change, for we find that the tem- 

 perature varies with the elimination of CO 2 , and urea, which give 

 a fair estimate of the chemical changes of the tissues. 



1. The diurnal changes in temperature are accompanied by an 

 afternoon increase and a morning decrease of CO 2 and urea. 



2. The tissue change giving rise to CO 2 decreases in a fasting 

 animal, as does also the production of heat. 



3. More CO 2 is eliminated after meals, when the temperature 

 also rises. 



4. The activity of various organs, such as the muscles and 

 glands, is associated with a local increase of temperature. 



INCOME AND EXPENDITURE OF HEAT. 



As repeatedly stated, the chemical changes which give rise to 

 the heat cause a certain waste of the tissues, which have again 

 to be renewed by the assimilation of various nutrient materials. 

 The food is thus really the fuel of the animal body, and the pe- 

 culiarity of this form of combustion is that the tissues assimilate 

 or convert into their own substance the fuel, and then themselves 

 undergo a kind of partial combustion, by means of which they 

 perform their several functions, amongst others heat-production. 



As already mentioned, heat is produced most abundantly in 

 those tissues which undergo most active chemical changes, hence 

 the protoplasmic cells of glands, and the contractile substance of 

 muscle must be looked upon as the chief agents in setting heat 

 free. 



The possible heat-income depends on the amount of nutrient 

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



