ELEMENTARY VITAL PHENOMENA 257 



heat is produced, for chemical processes are there present that are 

 accompanied by the production of kinetic energy, and heat is the 

 form of kinetic energy that is evolved in all such processes with- 

 out exception, either alone or in addition to other forms of energy. 

 In fact, there is even good ground for supposing with Pfliiger that 

 in single molecules of living substance temperatures of several 

 thousand degrees Centigrade become developed suddenly. This may 

 be the case in the production of a molecule of carbonic acid, since 

 the heat yielded by the combustion of carbon amounts to 8,000 

 calories. But the molecule of carbonic acid is excessively small, 

 and it is surrounded in the cell by an enormous number of other 

 molecules which possess a very low temperature. Hence, the 

 heat that suddenly flashes up is counterbalanced as rapidly as it 

 appears ; and, since all heat-forming molecules are not produced 

 simultaneously, but appear now here and now there between large 

 masses of other molecules, it is evident that the total temperature 

 of the cell resulting from the equalisation of all the various indi- 

 vidual temperatures cannot reach a remarkable height. Further, 

 with our crude methods of heat-measurement, we cannot yet 

 measure the actual heat given off to the outside by a single cell, 

 since the greater part is lost in the process by conduction and 

 radiation. It is, therefore, necessary to employ for the determin- 

 ation of the heat-production, not a single cell, but large cell- 

 complexes, such as considerable masses of tissue or whole 

 organisms. 



The production of heat is most evident in the bodies of homo- 

 thermal, or so-called warm-blooded, animals. It has already been 

 seen that the earlier division of animals into warm-blooded and 

 cold-blooded has been very fittingly replaced by that into homo- 

 thermal and poikilothermal animals, i.e., those that maintain 

 under all external conditions the same body-temperature and 

 those whose body-temperature rises and falls with the temperature 

 of the environment. Homothermal animals show most clearly the 

 production of body-heat because they have contrivances for storing up 

 heat in themselves to a certain definite degree and maintaining it 

 at this degree by an extremely delicate regulating mechanism. 

 Hence, with an external temperature not too high the body of the 

 homothermal animal is always warmer than the surrounding 

 medium. This may be determined readily by the method of 

 thermometric measurement. Thus, the body of man possesses in 

 its interior a constant temperature of 37 39 C., upon its surface 

 a temperature somewhat less, corresponding to the external 

 cooling, in the mouth-cavity about 37 C., and in the axilla about 

 36'5 C. Birds with their active metabolism have the highest 

 body-temperature, e.g., the swallow more than 44 C. But that 

 poikilothermal animals can attain considerable temperatures when 

 under conditions in which the heat produced by them is stored and 



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