662 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



hitherto it has not been found possible to trace any direct passage of these prolon- 

 gations into nerve-fibres. The commissure is formed entirely, according to Mr. 

 J. L. Clarke, by the fibrous portion of the gray matter, neither the anterior nor 

 the posterior columns having any direct connection by transverse fibres ; and 

 the central part of it is stated by him to consist of a layer of fine fibrous tissue 

 surrounding the wall of the spinal canal, which is lined with a layer of colum- 

 nar epithelium. The fibres of the gray substance and even those of the sub- 

 stantia gelatinosa are tubular, but are of extremely minute size, their average 

 diameter not exceeding l-10,000th of an inch, and some of them measuring as 

 little as 1-1 5,000th or l-16,000th. The course of the fibres which constitute 

 the roots of the Spinal Nerves, as seen in transverse sections of the Cord, is 

 thus described by Mr. J. L. Clarke. The bundles of fibres which form the pos- 

 terior root (Fig. 174, c, c) are much larger, but less numerous than those of the 

 anterior ; the fibres themselves are mostly finer and more delicate, their average 

 diameter being about l-7000th of an inch. On entering the posterior columns 

 of the Cord, the fasciculi traverse them obliquely inwards, interlacing and form- 

 ing with each other an intricate plexus. From this plexus, straight and distinct 

 bundles enter the posterior cornua along their whole breadth, and cross the 

 "substantia gelatinosa" both obliquely and at right angles; some being immedi- 

 ately continuous with fibres of the transverse commissure, some passing to the 

 vesicular tract, while others break up and form a finer network which extends 

 towards the anterior cornua. Some of these fibres, after traversing the gray 

 substance, pass out again into the posterior and lateral white columns ; and many 

 of those of the "substantia gelatinosa" seem to become longitudinal. The 

 fasciculi of fibres which constitute the anterior roots, on the other hand, traverse 

 the anterior columns of the Cord somewhat obliquely, and in straight and dis- 

 tinct bundles, which do not interlace with each other, but proceed directly to 

 the anterior gray cornu. On reaching this, they break up into smaller bundles 

 and separate fibres, which diverge in various directions; of those proceeding 

 towards the external border of the cornu, some pass out again into the antero- 

 lateral column, whilst others, after winding round groups of caudate vesicles, 

 curve inwards and join the fibres of the transverse commissure ; of those pro- 

 ceeding along the inner border of the cornu, a few pass into the white column 

 at the side of the median fissure, while others, entering the anterior portion of 

 the transverse commissure, cross to the opposite side ; and the remainder, plung- 

 ing into the central portion of the cornu, and winding among its vesicles, seem 

 to lose themselves in its substance, some of them probably changing their di- 

 rection and becoming longitudinal. According to Stilling, an absolute con- 

 tinuity is thus established between certain of the anterior and posterior root- 

 fibres of the same side ; and also between the anterior fibres of one side and 

 the posterior of the other, through the transverse commissure. The evidence 

 which he has adduced in favor of this doctrine, however, is by no means satis- 

 factory ; and for the present it must be regarded as hypothetical. That an 

 actual decussation of the fibres of the Cord is effected by means of the trans- 

 verse commissure, although denied by Hannover, seems to have been fully 

 proved by Mr. J. L. Clarke; this decussation, however, is limited, as already 

 stated, to the fibres of the gray matter. 1 



698. The Spinal Cord is by no means of uniform dimensions throughout its 



1 Mr. J. L. Clarke has succeeded in preparing transverse sections of the Cord of suffi- 

 cient thinness to enable them to be seen under high powers with transmitted light; 

 whereas the statements of Stilling, and of other observers who have used his method, 

 have been founded upon observations made upon comparatively opaque sections seen under 

 low powers, and often with reflected light, whereby the nature of the several structures 

 was often left in obscurity. 



