SPINAL CORD OF VERTEBRATA. 273 



roots into the anterior roots of other nerves above and below, both on the 

 same side and on the opposite. Of the portions of the roots which are con- 

 tinuous with the fibrous columns, the anterior would seem to have a connec- 

 tion with both the anterior and lateral columns ; and the posterior cannot be 

 said to be restricted to the lateral column, some of their fibres entering the 

 posterior division of the cord. 



348. If the white or fibrous portion of the Spinal Cord be really continu- 

 ous with the medullary matter of the Brain, the roots of the nerves which 

 enter it are in reality thus brought into connection with the Cerebral Hemi- 

 spheres and Cerebellum ; and the posterior division of these may, therefore, 

 be regarded as conducting to the Sensorium those impressions, which there 

 become sensations ; whilst the anterior roots convey the motive influence, 

 which has been propagated, by a voluntary or emotional impulse, down the 

 tract of the Spinal Cord with which they are continuous. On the other hand, 

 the passage of one portion of each set of roots through the grey matter of 

 the Cord, completes the nervous circle required for the performance of reflex 

 actions ; and by this they would seem to take place in Vertebrated animals, 

 just as through the distinct system of excito-motor fibres in the Articulata 

 ( 328.) The fibres which pass continuously from the posterior to the ante- 

 rior roots of the nerves on the same side, probably constitute the channel of 

 those reflex actions, which can be excited in apart supplied by any compound 

 nerve, by stimulating its afferent fibres, and thus causing a motor impulse to 

 be transmitted from the Spinal Cord along its afferent portion. The "fibres 

 which cross to the opposite side, will produce similar movements in its cor- 

 responding parts. And the fibres, if such there be, that pass from the pos- 

 terior (afferent) roots of each nerve, into the anterior (motor) roots of distant 

 nerves, would convey to a great variety of muscles, the influence of a stimulus 

 applied to a single afferent nerve. It follows, then, on this view of the cha- 

 racter of the Spinal Cord, that the continuity of the fibrous tracts is all that 

 is required, to convey the influence of the brain to the parts below ; whilst 

 the completeness of the nervous circle is all that is necessary, for the perform- 

 ance of reflex actions excited through it. This is found to be strictly true ; 

 the former having been observed in cases of disease, and the latter having 

 been proved by experiment. As far as simple reflex actions are concerned, 

 there is as much segmental independence in Vertebrata, as in the Articulata ; 

 but these actions seldom have so completely the character of adaptation, ami 

 are of a more irregular and convulsive nature. Still, however, there is an 

 essential correspondence between them ; and we may, therefore, regard the 

 distinction between the reflex and voluntary movements as the same in each 

 group ; the former predominating in Articulata ; the latter in Vertebrata. On 

 this view, then, each spinal nerve contains at least four sets of fibres. 



i. A sensory bundle passing upwards to the Brain. 



ii. A motor set, conveying the influence of volition and emotion down- 

 \vardsfrom the Brain. 



in. A set of excitor or centripetal fibres, terminating in the true Spinal 

 Cord or ganglion, and conveying impressions to it. 



iv. A motor or centifrugal set, arising from the same Ganglionic centre, 

 and conveying the motor impulse reflected_/V0w 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. 



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

 but it is now proved, 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 (infrequent oc- 

 currence, in which a portion of one of the columns has been almost entirely 



