126 FUNCTIONS OF THE NERVOUS SYSTEM. 



II. A motor set, conveying the influence of volition and emotion downwards 

 from the brain. 



III. A set of excitor or centripetal fibres, terminating in the true spinal cord 

 or ganglion, and conveying impressions to it. 



I V. A motor or centrifugal set, arising from the same ganglionic centre, and 

 conveying the motor impulse reflected/rom it to the muscles. 



Of these, the first and third are united in the posterior or afferent roots ; the 

 second and fourth in the anterior or efferent roots. 



167. It is difficult to trace the course of the fibres within the Spinal Cord; 

 and some recent experiments by Valentin, appear to prove, that Sir C. Bell 

 was not altogether correct in his idea that the functions of the columns of 

 the cord are respectively similar to those of the roots connected with them. 

 Cases, indeed, are of no unfrequent occurrence, in which a portion of one of 

 the columns has been almost entirely destroyed by injury ojjjlsease, without 

 any corresponding loss of the function attributed to it. Such cases have kept 

 alive, in the minds of many eminent practical men, a considerable distrust of 

 the accuracy of Sir C. Bell's conclusions. We have seen that, in regard to 

 the roots of the nerves, his first statements have been confirmed, and rendered 

 more precise, by subsequent researches ; but it is not so in regard to the func- 

 tions of the anterior and posterior divisions of the Spinal Cord. Bellingeri 

 was led, by experiments on the spinal cord, to the conclusion, that the anterior 

 roots of the nerves were for the flexion of the various articulations, and the 

 posterior for their extension. He also was wrong, in extending an inference, 

 founded on experiments on the Cord, to the roots of the nerves. The recent 

 experiments of Valentin, whilst they fully confirm Sir C. Bell's determination 

 of the functions of the roots of the nerves, coincide, to no small degree, w r ith 

 Bellingeri's opinion in regard to the offices of the anterior and posterior divi- 

 sions of the Cord. He obtained reason to believe that, in the Frog, neither 

 the superior nor inferior strand of the cord (posterior and anterior columns in 

 Man) solely possess motor functions ; but he found that, when the former were 

 irritated, sensations predominated ; and when the latter, motions were chiefly 

 excited. He further states that, if the superior strand (posterior column) be 

 irritated at the point at which the nerves of either extremity are given off, that 

 extremity is extended; and that if the inferior strand (anterior column) be 

 irritated, the extremity is flexed. At their entrance into the spinal cord, there- 

 fore, it would appear that the rrjotor fibres of the extensors pass towards the 

 superior stratum (posterior column in Man), whilst those of the flexors are 

 continuous with the inferior stratum (anterior column) ; their course being 

 more altered, however, when they are examined far from the point of issue. 

 This doctrine was confirmed by experiments on Mammalia ; and is borne out 

 (according to Valentin) by pathological phenomena observed in Man. Accord- 

 ing to this eminent physiologist, also, relaxation of the sphincters is analogous 

 to the extended state of the extremities ; and he has noticed a manifest relaxa- 

 tion of the sphincter ani in the frog, when the superior part of the spinal cord 

 was irritated so as to produce extension of the limbs. These statements are 

 entitled to considerable weight, on account of the quarter from which they 

 come ; but they are not, perhaps, to be received altogether without hesitation, 

 until confirmed by other physiologists, especially whilst the phenomena of 

 reflex action are still so imperfectly known. For it is quite possible that, 

 whilst stimulation of the anterior part of the cord may excite direct motions of 

 flexion, in preference to those of extension, the movements of extension pro- 

 duced by stimulating the posterior column may be of a reflex character. 



168. There is no reason to believe, that the functions of the Spinal Cord 

 are essentially different along its whole length. Everywhere it appears to 

 consist of a ganglionic centre, supplying nerves to its particular segment ; and 



