ANIMAL MOTION. 29 



in the large intestine, as Ed. Weber lias pointed out. Very 

 often, on the other hand, the will has no power over certain 

 muscles which, by their structure, and by the nature of the 

 nerves which animate them, belong to the system of the life 

 of relation. Habit, besides, by repeated exercise, appears to 

 extend the action of the will over the muscles, almost 

 indefinitely. The young animal shows, by the awkwardness 

 of his movements, that he is not in full possession of his 

 muscular functions; he seems to have to study the simplest 

 acts, and performs them badly ; while the gymnast, or the 

 skilled piano forte-player executes prodigies of agility, strength, 

 or precision, without any apparent effort of the will propor- 

 tionate to the result obtained. Many physiologists think, and 

 we are of the same opinion, that there exist in the brain, 

 and in the spinal marrow, centres of nervous action which 

 acquire certain powers, by force of habit. They attain to the 

 command and co-ordinal ion of certain groups of movements 

 without the complete participation of that portion of the brain 

 which presides over reasoning and the consciousness of our 

 actions. 



Let us lay aside these questions, which are still under inves- 

 tigation, and examine into the production of motion in a 

 voluntary muscle. The organ which generates motion is 

 composed of several elements. Simple as it is supposed to 

 be, it requires the intervention of muscular fibre, of the blood 

 vessels, which unceasingly convey to it the chemical elements 

 at whose expense the motion is to be produced, and finally, of 

 the nerve which excites motion in the fibre. 



When the physiologist desires to analyse the actions 

 which take place in the muscles, he does not deal, in the first 

 place, with voluntary motions, whose complexity is too great. 

 The operator isolates a muscle, and induces motion in it, by 

 bringing to act upon its nerve artificial excitements which he 

 has under his control. 



To give an idea of the part played by each of the elements 

 of the motive apparatus in the production of movemeit", it is 

 sufficient to operate upon the leg of a frog. By laying bare 

 and severing the sciatic nerve, the influence of will upon the 

 muscle may be suppressed, so that the latter will only execute 



