ORIGIN OF THE SPINAL NERVES. 391 



on the posterior root. The anterior root lies in contact with 

 the anterior surface of the ganglion, but none of its fibres in- 

 termingle with those in the ganglion. But immediately be- 

 yond the ganglion the two roots coalesce, and by the mingling 

 of their fibres form a compound or mixed spinal nerve, which, 

 after issuing from the intervertebral canal, divides into an an- 

 terior and posterior branch, each containing fibres from both 

 the roots (Fig. 141). 



According to Kolliker the posterior root-fibres of the cord 

 enter into no connection with the nerve-corpuscles in the gan- 

 glion, but pass directly through, in one or more bundles, which 

 are collected into a trunk beyond the ganglion, and then join 

 the motor root. From most, if not all, of the ganglionic cor- 

 puscles, one or two, rarely more, nerve-fibres arise and pass out 

 of the ganglion, in a peripheral direction, in company with the 

 posterior root-fibres of the cord. Each spinal ganglion, there- 

 fore, is to be regarded as a source of new nerve-fibres, which 

 Kolliker names ganglionic fibres. The destination of these 

 fibres is not yet determined : probably they pass especially 

 into the vascular branches of the nerves which they accompany. 



The anterior root of each spinal nerve arises by numerous 

 separate and converging fasciculi from the anterior column of 

 the cord ; the posterior root by more numerous parallel fasciculi, 

 from the posterior column, or, rather, from the posterior part 

 of the lateral column ; for if a fissure be directed inwards from 

 the groove between the middle and posterior columns, the pos- 

 terior roots will remain attached to the former. The anterior 

 roots of each spinal nerve consist exclusively of motor fibres ; 

 the posterior as exclusively of sensitive fibres. For the knowl- 

 edge of this important fact, and much of the consequent prog- 

 ress of the physiology of the nervous system, science is in- 

 debted to Sir Charles Bell. The fact is proved in various 

 ways. Division of the anterior roots of one or more nerves is 

 followed by complete loss of motion in the parts supplied by 

 the fibres of such roots ; but the sensation of the same parts 

 remains perfect. Division of the posterior roots destroys the 

 sensibility of the parts supplied by their fibres, while the power 

 of motion continues unimpaired. Moreover, irritation of the 

 ends of the distal portions of the divided anterior roots of a 

 nerve excites muscular movements; irritation of the ends of the 

 proximal portions, which are still in connection with the cord, 

 is followed by no effect. Irritation of the distal portions of 

 the divided posterior roots, on the other hand, produces ,no 

 muscular movements and no manifestation of pain ; for, as al- 

 ready stated, sensitive nerves convey impressions only towards 

 the nervous centres: but irritation of the proximal portions of 



