Chap, hi.] OF MOTEICITT. 393 



functionment. A large expenditure of motricity exhausts for a 

 time the nerves and the marrow. For them as for the muscles 

 it has been demonstrated that their chemical reaction, neutral 

 in the state of repose, becomes acid after labour. 



To simi up, the spinal marrow is an unconscious nervous 

 centre, a grand source of reflex acts. It probably holds under 

 its dominion a quantity of associated movements, harmonious 

 in appearance, but which in reality are for the most part effected 

 without the intervention of the conscious centres, of the cerebral 

 centres. This is why the young of many vertebrates run about 

 as soon as they are bom, when the cerebral hemispheres act 

 little or not at all, in any case cannot consciously command 

 complicated movements, which the young animal has not yet 

 learned to execute. 



The brain, on the other hand, is charged to accomplish reflex 

 acts of another order, which often trouble in their execution the 

 imconscious acts of the marrow. This is why this last nervous 

 centre acts with more energy and is more excitable when by 

 vivisection it has been in some sort delivered from the interven- 

 tion of its mobile neighbour. 



