THE STRUCTURE OF THE SPINAL CORD. 531 



or more simply, the ivhite commissure, or sometimes the anterior commissure 

 (Fig. 119, a. c.). 



If the section be taken at the level of the origin of a pair of spinal 

 nerves, it will be seen that the anterior or ventral root, piercing the white 

 matter opposite the head of the comma in several distinct bundles (Fig. 

 119, A. r.), plunges into the anterior cornu, while the posterior or dorsal 

 root (Fig. 119, P. r., P. r'.), having the appearance of a single undivided 

 bundle, passes, in part at least, into the posterior horn. Both roots are dis- 

 persed lengthways along the cord, the hinder roots of one nerve being close 

 to the foremost roots of the nerve below, but it is only the anterior roots 

 which are dispersed sideways. The compact bundle of the posterior root 

 divides, with tolerable sharpness, the white matter in each lateral half of 

 the cord into (1) a posterior portion lying between the posterior fissure and the 

 posterior root, which portion since, as we shall see, runs in the form of a col- 

 umn along the length of the cord, is called the posterior column, and (2) into 

 a portion lying to the outside of the posterior root between it and the ante- 

 rior fissure, called the antero-lateral column. This latter may be considered 

 as further divided, by the entrance of the anterior roots into a lateral column 

 {Fig. 119, fat. col.) between the posterior root and the most external bundle 

 of the anterior root, and into an anterior column (Fig. 119, ant. col.), between 

 the anterior fissure and the most external bundle of the anterior root. The 

 part traversed by the bundles of the anterior root, as they make for the 

 anterior horn, accordingly belongs to the anterior column ; but some writers 

 speak of the anterior column as lying between the anterior fissure and the 

 nearest bundle of the anterior root, thus making the region of the anterior 

 root belong to neither anterior nor lateral column. And indeed the dis- 

 tinction between the anterior and the lateral column is. to a great extent, 

 artificial. 



476. The " white matter " consists exclusively of medullated fibres 

 supported partly by connective tissue and partly by a peculiar tissue known 

 as neuroglia, of which we shall presently speak. The fibres are of various 

 sizes, but many of them are large, and in all of them the medulla is conspic- 

 uous. They run for the most part longitudinally, so that in transverse sec- 

 tions of the cord nearly the whole of the white matter appears under the 

 microscope to be composed of minute circles, the tranverse sections of the 

 longitudinally-disposed fibres, imbedded in the supporting structures. The 

 "" gray matter " also contains medullated fibres, but these are for the most 

 part exceedingly fine fibres possessing a medulla which appears to differ 

 from that of an ordinary nerve-fibre, since it does not stain readily with 

 osmic acid, but is rendered visible by special modes of preparation such as 

 that known as Weigert's. Hence these fine fibres are not apparent in ordi- 

 nary carmine or other specimens, and indeed their presence was for a long 

 time overlooked. Besides these fine medullated fibres, if we may call them 

 such, the gray matter contains what the white matter does not, nerve-cells 

 with branching processes, naked axis-cylinders, and delicate filaments aris- 

 ing from the division of axis-cylinders or from the branching of nerve-cells, 

 all these various structures being imbedded in neuroglia. Owing to the re- 

 lative abundance of the white refractive medulla, the white matter possesses 

 in fresh specimens a characteristic, opaque white color ; hence the name. 

 The gray matter from the relative scantiness of medulla has no such opaque 

 whiteness, is much more translucent, and in fresh specimens has a gray or 

 rather pinkish-gray color, the reddish tint being due to the presence partly 

 of pigment and partly of blood, for the bloodvessels are much more abun- 

 dant in the gray matter than in the white. 



The pia mater which closely invests the cord all around consists of con- 



