LP. xvi.] PHYSIOLOGICAL OFFICE OF RESPIRATION. 253 



mechanical in the labour of the philosopher who reflects, of 

 the literary man who writes, of the musician who composes. 

 ,These effects, considered as purely moral, yet possess some 

 physical and material attribute, which permits us to compare 

 ithem with the efforts of the labouring man. It is therefore not 

 iwithout justice that the French language has included under the 

 general denomination of labour, the efforts of the mind as well as 

 i those of the body, the labour of the cabinet and the labour of the 

 artisan, "i 



Truly it is singular and regrettable that, even in our days, and 



: in spite of the brilliant triumph of the doctrine of the correlation 



I of the physical forces, no physiologist has brought this beautiful 



'idea of Lavoisier into the domain of facts, rigorously and 



i minutely observed. The last champions of vitalism and animism 



always repeat that thought escapes the law of the correlation of 



i physical forces, that its mechanical equivalent cannot be deter- 



; mined. Now it is incontestable that every cerebral activity 



answers to an absorption of oxygen, which manifests itself by 



a larger exhalation of carbonic acid, and by the renal excretion 



of certain products of oxydation. It would surely be possible 



to estimate approximately in calories, and consequently in kilo- 



grammetres, this process of oxydation. Thus the mechanical 



equivalent of thought, and even of the different modes of 



thought, would be determined. 



In vertebrated animals with lungs, the movement of the venti- 

 lating apparatus of the lungs, of the thoracic frame, are generally 

 executed automatically ; but they are much more subject than the 

 movements of the heait, to the will, which can accelerate, retard, 

 or suspend them ; it is, then, natural that certain lesions of the 

 nervous centres immediately affect, not directly indeed the ex- 

 change of gases on the pulmonary surface, but the alternate 

 movements of the thoracic frame. In effect, certain wounds of 

 the spinal marrow, in rabbits and guinea-pigs, cause a slackening 

 of the respiration, and a gradual coldness ; in a word, a state very 

 1 Lavoisier, Memoires de V Academic des Sciences, 1789. 



