76 



THE HUMAN MECHANISM 



body with its dendrites and axons, is an anatomical unit 

 a cell. To this entire cell the term " neurone " is given. The 

 neurone is the cellular unit of the nervous system, just as 

 the muscle fiber is the cellular unit of the muscle, and the 

 gland cell of the gland. 



6. Fibers of the dorsal, or posterior, roots. The ventral 

 roots, as we have seen, are entirely efferent in function ; 

 that is, they conduct impulses only away from the spinal 



FIG. 43. Semidiagrammatic longitudinal section of a ganglion of the dorsal 



(posterior) root 



cord. The dorsal, or posterior, roots, on the other hand, are 

 found to be essentially afferent (Latin ad, " to " ; ferre, " to 

 carry " ) ; that is, they carry impulses from outside toward and 

 into the spinal cord. This is shown by the fact that when 

 these roots are destroyed by disease, muscles can still be 

 thrown into contraction, glands will still secrete, etc., 

 that is, there is no interference with efferent impulses, 

 but no sensations are received from the part of the body 

 to which these nerves are distributed; pinching the skin is 

 riot felt; the flesh may be burned and its owner suffer no 



