NERVOUS SYSTEM OF VERTEBRATA. 



Structure of the Spinal Cord, ac- 

 cordingto Stilling; A, posterior fibres 

 continuous with the anterior of the 

 same side, through the nucleus of 

 the cord; B, posterior fibres continu- 

 ous with the anterior of the opposite 

 side. 



terior columns only, and that the posterior are Fig. 19. 



restricted to the lateral columns. On the other 



hand, Mr. Grainger and Mr. Swan maintain that 



both sets are connected with the lateral cclumns 



only ; the anterior and posterior lateral fissures 



definitely limiting the two roots. Perhaps both 



these statements are rather too exclusive. The 



anterior roots would seem to have a connection 



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. 



166. As the white or fibrous portion of the 

 Spinal Cord Jkcontinuous with the medullary 

 matter of theJBln,the roots of the nerves which 

 enter it are "flfcity thus brought into connec- 

 tion with the Cerebral Hemispheres and Cere- 

 bellum ; and the posterior division of these may, 

 therefore, be regarded as conducting to the brain 

 those impressions which there become sensa- 

 tions ; whilst the anterior roots convey the motive 

 influence, which has been propagated, by a volun- 

 tary 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 gray matter 

 of the Cord, completes the nervous circle re- 

 quired 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 ( 143). The fibres which pass continu- 

 ously from the posterior to the anterior roots of the nerves on the same side, 

 probably constitute the channel of those reflex actions, which can be excited 

 in a part 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 efferent portion. The fibres which cross to the opposite side, will produce 

 similar movements in its corresponding parts. And the fibres, if such there 

 be, that pass from the posterior (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 character 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 performance 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, and 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 tte same 

 in each group ; . the former predominating in Articulata, the latter in Verte- 

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

 fibres. 



I. A sensory bundle passing upwards to the brain. 



11* 



