52 PHYSIOLOGY CHAP. 



thermic balance is undisturbed, the average temperature of the 

 body remains the same, and is about equal to the difference 

 between the production of heat from the combustion of the food 

 and the loss of heat to the environment. 



The energy set free during the process of oxidation takes 

 other forms besides that of heat, more especially that of work. 

 When, however, this work is not transmitted outwards but is 

 expended in the interior of the organism, as for instance in the 

 case of the heart, the whole amount of work is transformed into 

 heat and can be measured as such. 



When we know the amount of the exchange of material, we 

 can calculate with considerable accuracy from the heat of com- 

 bustion of the food the total amount of heat produced, taking 

 into account the heat equivalent of the external work. Conversely, 

 by determining the amount of heat produced, inclusive of external 

 work, we can calculate almost exactly the total exchange of 

 material, without, however, being able to draw any conclusions 

 as to the consumption of individual food substances. 



I. In olden days scientists believed in the existence of an innate 

 heat upon which life was dependent ; the heart was regarded as 

 the source from which this heat was derived and from which it 

 was diffused throughout the body by means of the waves of the 

 blood in the arteries. Haller (1760) was the first to point out 

 the inadequacy of this theory by the observation that the heart 

 was no hotter than the intestines or other organs. Boerhaave put 

 forth a mechanical theory of thermogenesis ; he regarded the heat 

 as a result of the friction of the blood flowing in the blood- 

 vessels, but Haller showed this theory also to be inadequate by 

 reference to cold-blooded animals. After mentioning the 

 hypothesis of electric action and fermentation of the blood, he 

 expressed his personal opinion as follows : " Hactenus certe 

 rnaxime videtur, utique a motu sanguinem incalescere, etsi 

 nondum constat, quare magis quam aqua, et quare non super 

 certum gradum incalescere possit." 



Not until after Lavoisier (1789), with the assistance of his 

 fellow-workers Laplace and Seguin, had developed his theory of 

 respiration, which he regarded as a slow combustion of the carbon 

 and hydrogen in the animal body, can science be said to have 

 been on the right track leading to a true theory of thermogenesis 

 (see Vol. I. Chap. XI. 2). Lavoisier was, however, wrong in suppos- 

 ing that the heat generated during the combustion of organic 

 compounds was equal to that resulting from the combustion of their 

 elements. Favre and Silbermann (1852) were the first to prove 

 by numerous experiments the inaccuracy of this supposition, and 

 thus cleared the way for a further development of our knowledge 

 of the heat of combustion of the organic substances of food and 

 the different constituents of which our organism is composed. 



