ANIMAL HEAT. 



19 



fact, while it is relatively easy to estimate the work of our 

 muscles when employed in lifting a burden, there are other 

 muscular actions which constitute an important sum of work 

 and which we do not yet know how to value with precision ; 

 we allude to the movements of the circulation, and especially 

 to those produced by the breathing apparatus. 



The remarks which we have made upon the greater number 

 of the physiological experiments from which it has been sought 

 to establish numerical data, apply to that of Hirn. But 

 though it cannot furnish an exact determination, this ex- 

 periment at least enables us to perceive the manner in which 

 the phenomena vary ; it shows that a . certain quantity of heat 

 always disappears from the organism when external work 

 is produced. No greater precision could be obtained in the 

 measure of thermo -dynamic transformation in the greater 

 number of steam-engines, and yet nobody disputes that in 

 these motors heat and work are substituted for one another in 

 equivalent relations. 



CHAPTEK IIL 



ON ANIMAL HEAT. 



Origin of animal heat— Lavoisier's theory — The perfecting of this theory 

 — Estimates of the forces contained in aUments, and in the secreted 

 products — Difficulty of these estimates — The force yielded by ali- 

 mentary substances is transformed partly into heat and partly into 

 work — Seat of combustion in the organism— Heating of the glands 

 and muscles during their functions— Seat of calorification — Interven- 

 tion of the causes of cooling — Animal temperature — Automatic regu- 

 lator of animal temperature. 



DuRiKG a long period, animal heat was considered to be of 

 a peculiar kind, distinct from that which is manifested in the 

 inorganic kingdom ; this arose from certain conditions under 

 w^hich the living tissues become hot or cold, without its 

 being easy to discover how this heat appears, or how it 

 disappears. It was almost natural to admit that heat is 



