CH. XIV.] 



SPINAL NERVE ROOTS 



155 



unless the axons reach it, it is ineffective in so far as any real new 

 formation of nerve-fibres is concerned. If, however, the axons reach 

 the peripheral segment, the work of the neurilemmal cells has not 

 been useless, for they provide the supporting and nutritive elements 

 necessary for their continued and successful growth. Moreover, the 

 neurilemmal activity appears to be essential. In the white fibres of 

 the central nervous system the neurilemma is absent; in this 

 situation not only is the removal of the products of degeneration a 

 very slow process, hut regeneration does not occur. 



Functions of the Roots 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 

 originates 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 multipolar 

 cells in that portion of the 

 grey matter in the interior 

 of the spinal cord which we 

 shall presently learn to call 

 the anterior horn. These 

 nerve-fibres are all medul- 

 lated ; 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 sym- 

 pathetic chain, which then 

 distributes non - medullated 

 fibres to the involuntary fibres of the blood-vessels and viscera. 



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

 of nerve-cells forming the spinal ganglion. Each nerve-cell is 

 enclosed within a nucleated sheath of connective-tissue, 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. 159, A), 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 periphery. In the adult mammal (not in 

 fishes) the two processes coalesce in the first part of their course, 

 forming a ^-shaped junction (fig. 159, B). 



B. 



FIG. 159. A, bipolar cell from spinal ganglion of a 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.) 



