Physiology of Nerves and Nerve- cells. 509 



tire and painless freedom of motion, as regards all parts of the hand. 

 .My hand is now open, or it is shut,' they say. ' I touch the thumb 

 with the little finger,' &C." 1 



Every nerve fibre, while in its normal position in the body, excites in 

 its receiving organ only one sort of action, no matter how the nerve 

 may be excited. Thus the irritation of the optic nerve produces the 

 sensation of light even when it is severed from the eye, provided its at- 

 tachment to the brain is not disturbed. If a nerve be cut in two, its 

 conducting power over the cut is gone, even if the two ends of the 

 nerve are placed together. But a cut or bruise in a nerve is repaired 

 by the natural healing process. Even if a piece of nerve as long as two 

 or two and a quarter inches, be removed, it will be renewed. The re- 

 pair begins by the nerve matter at one of the cut ends, forming into a 

 grayish bunch and pushing along the former track of the nerve till it 

 unites with the opposite end. To perfect such repair requires from 

 four to six months. At first, the connecting section is more slender 

 than the original, but by degrees it gets its proper size, and peculiar 

 whitish color. (Papillon.) 



According to Landois, the repair of a nerve depends on its connec- 

 tion with its central ganglion. This is the spinal cord, in the case of 

 the motor nerves, and the ganglion of the posterior root, in the case of 

 a spinal sensory nerve. These seem to Be the headquarters for the 

 nourishment as well as centers of the activities of their respective nerves, 

 and repairs are propagated ( chiefly ) from them. Nerves have been 

 diverted and given new connections. This is readily accomplished when 

 a motor nerve is made to take the place of another motor nerve, or a 

 sensory nerve of another of the same function ; and it is held by some 

 authorities to lie possible even to connect a motor to a sensory nerve. 



The size of nerve fibres in different animals, according to Todd's 

 measurement, is as follows : 



Fishes Eel, -^^ of an inch in diameter. 



Reptiles Frog, -- to ^ 



Birds > 



Mammals, to 



These are for the final fibres which do not divide or branch. ( Owen.) 



1 Ferrier, Functions of Brain, 388. 



