ORGANS OF THE BODY. 575 



before and behind, the anterior (a) and posterior medium fissures (b), so 

 that its two halves are only connected at the bottom of the anterior fissure 

 by a white band (&), the white commissure or commisura anterior. The 

 isthmus, however, contains besides a band of grey substance, known as 

 the posterior commissure (I). The white matter of the cord may be con- 

 sidered as consisting of three imperfectly defined symmetrical longitudinal 

 bands, the anterior (g), lateral (h), K&& posterior (i) columns. 



In the cervical portion of the spinal marrow the latter most internal 

 and posterior portion constitutes what is known as the band of Gott, to 

 which we will again refer in speaking of the medulla oblongata. 



At the junction of lateral and anterior columns the motor roots of the 

 spinal nerves penetrate as far as the anterior cornu ; while the entrance 

 of the posterior sensory roots takes place in a similar manner at the point 

 of union of the middle and hinder columns. 



Looking at it from a histological point of view, the whole spinal marrow 

 may be said to be supported interstitially by a lowly-organised vascular 

 connective-tissue, and to be composed of nervous fibres and ganglion cells 

 imbedded in this framework. In the white substance, however, we find 

 fibrous nerve-elements alone, but in the grey, besides these, ganglion 

 cells. There are, however, so many difficulties still connected with the 

 investigation of the more minute arrangement and combination of these 

 nerve-elements, that, with the brain, the spinal cord may be said to be one 

 of the most obscure and unsatisfactory fields of modern histological 

 research. One of the obstacles to advance in this direction is, that we 

 are unable here to draw any sharp line of distinction between nervous 

 and connective-tissue constituents (see 119). One school of histologists 

 believe that connective-tissue constitutes a very large portion of the sub- 

 stance of the spinal cord, while quite the opposite view is held by another 

 party. 



REMARKS. (1.) Literature is very rich in treatises on the structure of the spinal 

 cord. Besides numerous Continental essays by Stilling and Wallach, Schroder van 

 der Kolk, Koelliker, Reissner, Deiters, Gcrlach, may be mentioned those of Lockhard, 

 Clarke, Philos. Transact. 1851, p. ii. p. 607, and p. iii. p. 347; and Beale's Archiv. of 

 Medic. 1858, p. iii. p. 200. Further, in the Proceed, of Roy. Soc. vol. viii. No. 27 ; and 

 Philos. Trans. 1858, p. i. p. 231, and 1859, p. i. p. 437. /. Dean's Microscopical 

 Anatomy of Lumbar Enlargement of the Spinal Cord, Cambridye (U.S.) 1861. W. 

 Hendry in Micros. Journ. 1863, p. 41. 



292. 



We shall now consider the neuroglia or connective-tissue sustentacular 

 substance of the spinal cord, whose chief peculiarities have been already 

 touched on in a former section ( 119). 



In it we have a framework, as it were, for the medulla, in contact with 

 the pia mater externally, and continuous throughout the whole cord, 

 though of by no means of the same structure in the different divisions of 

 the latter. 



We find it in its simplest form surrounding the central canal as a ring 

 merging imperceptibly at its periphery into the grey matter. To this 

 several names have been given, such as " central ependymal thread," " grey 

 central nucleus," "gelatinous central substance." It presents itself here 

 as a soft substance of homogeneous, streaky, or even at certain points 

 finely fibrous appearance. Filiform processes from the epithelial cells of 

 the axis canal project into it, as also connective-tissue ramifications of the 

 pia mater from both fissures of the cord. Cellular elements may also be 



