620 THE FUNCTION OF THE SPINAL CORD 



regard. In 1822 Magendie 1 succeeded in demonstrating that the 

 division of the anterior roots destroys motion, while the section of the 

 posterior roots produces a loss of sensation. Owing, however, to the 

 fact that the former is in possession of a perfectly local system 

 of sensory fibers and that the latter is connected with motor reflex 

 paths, this investigator did not succeed in fully establishing their 

 function. This end was finally attained by Joh. v. Miiller as a result 

 of his experiments upon the spinal roots of the frog. 



In its modern form the Bell-Magendie law holds that the afferent 

 impulses from the superficial and deep parts of the trunk and ex- 

 tremities are conducted into the cord by way of the posterior roots, 

 while the efferent impulses to these parts leave this structure over the 

 fibers of the anterior roots. Thus, a most perfect localization of 

 sensory and motor function is had in this region of the nervous system. 

 To prove this, we may resort to the methods of division and stimulation, 

 as follows: 



1. Section: 



(A) Anterior root: loss of motion in the regions supplied by this nerve. 



(B) Posterior root: loss of sensation (contralateral) in the region innervated by 

 this nerve (ataxia and loss of reflex movements). 



2. Stimulation: 



(A) Anterior root: 



(a) Distal end : motor results of the kind ordinarily produced by this nerve. 



(b) Central end : no motor results, but retrogressive sensation. 



(B) Posterior root : 



(a) Distal end : no results. 



(b) Central end : sensations of the kind ordinarily conveyed by this nerve. 



In its complete form the Bell-Magendie law also takes cognizance 

 of certain minor facts which are as follows: It has been mentioned 

 above that the anterior root as such is not devoid of sensation and 

 hence, does not differ in this regard from other tissues. For this 

 reason its excitation must give rise to "retrogressive" sensory impulses 

 which are purely local in their origin, and should therefore be sharply 

 differentiated from those which arise at the periphery and attain 

 consciousness by way of the posterior group of fibers. 2 In the second 

 place, as the sensory impulses traversing the posterior roots also in- 

 clude those which help in the formation of the muscle-sense, the divi- 

 sion of these fibers must necessarily be followed by a certain degree 

 of ataxia; in other words, in the absence of the sensory impressions 

 from the muscles and tendons, the muscular movements must lose 

 their coordinated character. In the third place, it should be re- 

 membered that the stimulation of the central end of the divided 

 posterior root may also give rise to movements, but these occasional 

 motor effects are reflex in their nature and cease immediately after the 

 division of the anterior roots. Being desirous of simplifying this topic 

 as much as possible, I refrain at this time from a discussion of certain 



1 Jour, de Physiol., ii, 1822, 276. 



2 Cl. Bernard, Lemons sur la physiol. et la path, du syst. nerveaux, i. 



