168 



PHYSIOLOGY OF NERVE 



[CH. XV. 



exclusively central origin. This view I thoroughly endorse. Mott 

 and I have also shown that when the regenerated fibres are again 

 cut, degeneration takes place in the peripheral direction only, and 

 this is a strong piece of evidence that growth had not started from 

 the periphery centralwards, for the direction of nutritive control is 

 always the direction of growth. 



The manifest activity of the neurilemmal cells is probably largely 

 nutritive rather than formative, but is nevertheless of great impor- 

 tance, for, in situations like the central nervous system, where the 

 neurilemma does not exist, 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- 



B. 



Fi . 187. A, Bipolar cell from spinal ganglion of a 4J 

 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 



are 



lated ; the large ones join 

 up with the posterior root 

 to form the spinal nerve ; 

 the small nerve-fibres leave 



cell from the spinal ganglion of the" adult ; the two the 1'OOt and paSS to the Sym- 

 processes have coalesced to form a T-shaped junction, imthetic chain which tlieil 

 (Diagrammatic.) -r 



distributes non - medullated 



fibres to the involuntary muscular 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 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. 187), 

 one of which grows to the spinal cord, where it terminates by 

 branching around the multipolar cells of the grey matter ; the other 



