THE STRUCTUEE OF THE SPINAL COED. 545 



fibres of a spinal nerve soon after entering in or before issuing from the 

 spinal cord are connected with nerve-cells lying in the neighborhood of the 

 attachment of the nerve to the cord. We should, we say, expect to find 

 this ; but owing to the difficulty of tracing individual nerve-fibres through 

 the tangled mass of the substance of the cord, our actual knowledge of the 

 termination of the fibres of the posterior root and origin of the fibres of the 

 anterior root is at present far from complete. 



With regard to the anterior root, there can be no doubt that a very large 

 proportion of the fibres in the root are continuations of the axis-cylinders of 

 cells in the anterior horn. The fibres which can thus be traced are of large 

 diameter and appear to be chiefly if not exclusively motor fibres for the 

 skeletal muscles. In the frog a laborious enumeration, on the one hand, of 

 the number of fibres in the anterior roots, and, on the other hand, of the 

 number of cells of the anterior horn in the areas corresponding to the 

 nerve-roots has, it is true, shown a very remarkable agreement in the number 

 between the two. We might be inclined from this to conclude that all the 

 fibres of an anterior root start directly from cells in the anterior horn, and 

 that all the cells in the anterior horn end in fibres of the nearest anterior 

 root, But several considerations prevent us from trusting too much to this 

 observation especially in the case of the higher animals. The anterior root 

 contains other fibres than motor fibres for the skeletal muscles, vasomotor 

 fibres for instance, secretory fibres and others ; and it is a priori unlikely 

 that these should have origin from the same cells as the motor fibres of the 

 skeletal muscles. Moreover, as a matter of fact, some of the fibres have 

 been traced through the anterior horn, on the one hand, toward the posterior 

 horn, and, on the other hand, toward the lateral column ; others again are 

 found to pass through the anterior horn of their own side to the bottom of 

 the anterior fissure where, crossing over to the other side and thus forming 

 part of the anterior white commissure, they appear to ascend to the anterior 

 horn of the other side. We cannot at present make any positive statement 

 as to the real origin and exact nature of these fibres which thus upon enter- 

 ing the cord pass by the cells in the anterior horn without joining them, 

 though those which cross by the anterior white commissure are supposed to 

 take origin in the cells of the anterior horn of the other side ; it is sufficient 

 for our present purposes to remember that while a large number of the fibres 

 of the anterior root, presumably those supplying the skeletal muscles, take 

 origin in the cells of the anterior horn, shortly before they issue from the 

 cord, others have some other origin. -And similarly we have reason to think 

 that all the cells in the anterior horn do not send out axis-cylinder processes 

 to join the anterior roots of the same side. We may, however, regard a 

 large number at all events of the cells of the anterior horn, at the level of 

 as well as a little below and a little above the level of the exit of any par- 

 ticular anterior root, as constituting a sort of nucleus of origin for the larger 

 number of the fibres, and those most probably the skeletal motor fibres, of 

 that anterior root. 



The posterior root enters the cord not in several bundles laterally scat- 

 tered as does the anterior root, but in a more compact mass. This mass, 

 however, consists of at least two distinct bundles, which upon their entrance 

 into the cord, take different courses. One bundle, the larger one, lying to 

 the inner or median side of the other, consisting of relatively coarse fibres, 

 and called the median bundle (Fig. 121, JV), passes obliquely into the lateral 

 part of the external posterior column, which, as we have said, is in conse- 

 quence often spoken of as the posterior root-zone. Here the fibres changing 

 their direction run longitudinally for some distance upward (some, however, 

 certainly in the upper cervical region, and probably in other regions, run a 



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