CH. XV.] 



ROOTS OF THE SPINAL NERVES. 



169 



cut fibres and the cells from which they originate undergo slow 

 atrophic changes (disuse atrophy). 



Functions of the Hoots of the Spinal Nerves. 



The general truths enunciated in the two preceding sections 

 are well illustrated by the experiments made to determine the 

 functions of the roots 

 of the spinal nerves. 

 Each spinal nerve ori- 

 ginates from the spinal 

 cord by two roots. One 

 of these is called the 

 anterior or ventral root : 

 it consists of nerve- 

 fibres which originate 

 from the large multi- 

 polar cells in that por- 

 tion of the grey matter 

 in the interior of the 

 spinal cord which we 

 shall presently learn to 

 call the anterior horn. 

 These nerve-fibres . 



Fig. 180. A, Bipolar cell from spinal ganglion of a 



B. 



4i weeks embryo, n, nucleus ; the arrows indicate 

 the direction in which the nerve processes grow, 

 one to the spinal cord, the other to the periphery. 

 B, a cell from the spinal ganglion of the adult ; the 

 two processes have coalesced to form a T-shaped 

 junction. (Diagrammatic.) 



are 



all medullated : the 

 large ones join up with 

 the posterior root to 

 form the spinal nerve ; 

 the small nerve-fibres 



leave the root and pass to the sympathetic chain, whence they are 

 distributed as non-medullated fibres to the involuntary muscular 

 fibres of the blood-vessels, etc. 



The other root, the posterior or dorsal root, has upon it a col- 

 lection of nerve-cells forming the spinal ganglion. Each nerve- 

 cell is enclosed within a nucleated sheath of connective tissue 

 origin, and it is from these nerve-cells that the fibres of the 

 posterior roots grow. In the embryo, each nerve-cell has two 

 processes (fig. 180), one of which grows to the spinal cord, 

 where it terminates by branching around the multipolar cells of 

 the grey matter ; the other process grows outwards to the peri- 

 phery. In the adult mammal (not in fishes) the two processes 

 coalesce in the first part of their course, forming a 2'-shaped junction. 



The first experiments on the functions of the spinal nerve-roots 

 were performed in this country by Sir Charles Bell ( 1 8 1 1 ), and in 



