CHAP, i.] THE SPINAL CORD. 877 



skeletal muscles, vaso-motor 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 towards the posterior 

 horn and on the other hand towards 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 com- 

 missure, 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 

 entering 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 particular 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 D 

 laterally scattered 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. 98, P.r'), passes obliquely into 

 the lateral part of the external posterior column, which, as we 

 have said, is in consequence often spoken of as the posterior 

 root-zone. Here the fibres changing their direction run longi- 

 tudinally for some distance upwards (some however, certainly in 

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

 short distance downwards) but eventually either go, as we shall 

 see, to form the median posterior tract or make their way back 

 into the grey matter at the base of the posterior horn and thus 

 join the vesicular cylinder, though some are said to be continued 

 on through the grey matter into the anterior horn. The other 

 smaller bundle placed to the outside of the former, and called the 

 lateral bundle (Fig. 98, P.r), may be again divided into an inter- 

 mediate bundle (Fig. 99, Pr) lying next to the median bundle, 



