496 THE NERVOUS SYSTEM. 



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 nume- 

 rous 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 posterior 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 know- 

 ledge of this important fact, and much of the consequent 

 progress of the physiology of the nervous system, science 

 is indebted 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 sensa- 

 tion 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 unim- 

 paired. 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 

 already stated, sensitive nerves convey impressions only 

 towards the nervous centres : but irritation of the proximal 

 portions of these roots elicit signs of intense suffering. 

 Occasionally, under this last irritation, muscular move- 

 ments also ensue ; but these are either voluntary, or the 

 result of the irritation being reflected from the sensitive to 

 the motor fibres. Occasionally, too, irritation of the distal 



