LECT. XIV. XV. PRODUCTION OF NERVOUS FORCE. 291 



lo wed by a loss of power, and as we see the animal machine 

 recover its aptitude for exercise, after having obtained food 

 and rest, we must admit that the force necessary to muscu- 

 lar action may arise from the chemical actions of nutrition ; 

 inasmuch as, by means of the latter, and of repose, this 

 force is reproduced and accumulates in the nervous sys- 

 tem. Interrupt for a certain time the sanguineous circula- 

 tion in a muscle, and soon this becomes incapable of con- 

 tracting; but with the return of blood the muscular force 

 revives. In animals, where circulation and respiration are 

 very active, the development of muscular force is more 

 considerable. , 



But among the numerous chemical actions that occur in 

 animals, which is the one that gives rise to the force which 

 is put in action during muscular contraction ? It is impos- 

 sible to give a satisfactory answer to this question. 



Physiologists now admit that heat is produced by the 

 combustion of fatty matters, and principally by that of 

 bodies into which fecula is transformed during digestion: 

 the nervous force may be due to chemical actions which 

 take place during the changes which the neutral azotized 

 substances of the tissues undergo. But I know of no ex- 

 periment which directly proves this difference of origin, 

 between heat and the nervous force. 



Of all chemical actions of which the animal is the seat, 

 the only one which we perfectly know, and which we 

 have even measured, is that which produces carbonic acid. 

 On the average, man converts and exhales, in the form of 

 carbonic acid, 10 to 15 grammes of carbon per hour. 



Setting out with these data, we shall endeavour to com- 

 pare the nervous force which results from this chemical 

 action, by representing the mechanical work done by a 

 man in the space of one day, with the quantity of work that 

 this same action could produce in the same space of time, 



