CHAP, ii.] OF CELLS AND OF NERVOUS FIBRES. 887 



by the motory cells of the peripheric nervous cords, this com- 

 mand arrives at the muscles charged to execute it. The cycle is 

 then complete, and the mechanical stimulation of the extremities 

 of some sensitive nervous fibres has, like a succession of gun- 

 discharges, determined a sensation, a ratiocination, a volition, 

 movements. 



Such is the complete series, but it is not always so distinct. 

 The one or the other, even the one and the other of the two first 

 periods, may be lacking. That depends on the organisms and 

 the organs. Assuredly each of the three stages of this nervous 

 circulation desires a small special study. We have therefore 

 now to occupy ourselves with motricity, with sensibility, and 

 with thought. 



c c 2 



