RELATIONS OF ANIMAL HEAT TO RESPIRATION. 429 



acid, and that the tissues themselves have the property of 

 appropriating oxygen and exhaling carbonic acid, those who 

 adopt the theory of Lavoisier have simply changed the seat 

 of oxidation from the lungs to the general system. 



It has been proven beyond question that oxygen, of all 

 the principles introduced from without, is the one most im- 

 mediately necessary to nutrition; and it differs from the 

 class of substances ordinarily known as alimentary, only in 

 the fact that it is consumed more promptly and constantly. 

 In the same way, carbonic acid is to be regarded as an ele- 

 ment of excretion, like urea, creatine, etc., differing from 

 them only in the immediate necessity for its elimination. 1 

 As the comparatively slow excretion of urea and other nitro- 

 genized matters is connected with the ingestion of ordinary 

 alimentary substances that are slowly appropriated by the 

 tissues, so the rapid elimination of carbonic acid is connected 

 with the equally rapid appropriation of oxygen. There is 

 no reason why we should not regard carbonic acid, like other 

 effete substances, as an excretion, the result of disassimila- 

 tion of the tissues generally ; but, more closely than any, it 

 is connected with the rapid and constant evolution of heat. 

 This view is proven by the experiments of Spallanzani, 3 

 W. F. Edwards, 3 and Collard de Hartigny. 4 All of these 

 eminent observers demonstrated, beyond a doubt, that car- 

 bonic acid may be formed in the system and exhaled, in 

 animals deprived of oxygen, and that its exhalation will 

 take place from a piece of tissue freshly removed from a 



1 Collard de Martigny, who was one of the most powerful opponents of 

 the combustion-theory of animal heat, concludes the account of his experi- 

 ments on the production of carbonic acid with the statement that it " is a prod- 

 uct of assimilative decomposition, secreted hi the capillaries, and excreted by the 

 lungs" (Journal de physiologic, Paris, 1830, tome x., p. 161). 



8 SPALLAXZAXI, Jfemoires sur la respiration, Geneve, 1803, pp. 86, 343. 



3 EDWARDS, De linfluen.ee, des agens physiques sur la vie, Paris, 1824, p. 

 443, et seq., and p. 455, et seq. 



* COLLARD DE MARTIGNY, Recherches experimentales et critiques sur fabsorption 

 et sur F exhalation respiratoires. Journal de physiologic, Paris, 1830, tome x., p. 

 124. 



