Chap, xvi.] PHYSIOLOGICAL OFFICE OF EESPIEATIOIT. 253 



the meclianical 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 

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

 without justice that the French language has included under the 

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

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

 artisan."! 



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

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

 of the physical forces, no physiologist has brought this beautiful 

 idea of Lavoisier into the domain of facts, rigorously and 

 minutely observed. The la.st champions of vitalism and animism 

 always repeat that thought escapes the law of the correlation of 

 physical forces, liat 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 heart, to the will, which can accelerate, retard, 

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

 nervous centres immediately afiect, not directly indeed the ex- 

 change of gases on the pulmonary smface, but the alternate 

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

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

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

 1 Lavoisier, Mimoives de VAccuUmie dea Seiences, 1789. 



