7V//: S/7V.I/. CORD. 



CG9 



the two median lis-mvs mentioned in de-cribini: tin- exterior of the coid, 

 the inferior of which is wider and deeper than that of the Mij.rior, whose 

 situation is scarcely perceptible. 



These two fissures advance one before the other, and do not me.-t 

 so as to completely divide the 

 cord into two lateral halves, but 

 n separated by two thin 

 horizontal and superposed bands 

 of nervous matter, that pass from 

 one end to the other of the me- 

 dullary axis. The inferior, formed 

 of white substance, corresponds to 

 the bottom of the inferior fissure ; 

 while the superior, composed of 

 grey matter, meets the superior 



SECTION OF Till: Sl'IXAI, COUD OF Till: IIOKSi: 

 AT Till: U'MIi.Vli KKMON: MA'lMHI.Ii TWO 

 DIAMETERS. 



1, Su[>cri'jr inu<lian fissure; 2, Interior iiK-li.-m 

 fyShie; 3, 3, Superior collateral fissun-.-: 

 4, 4, Inferior ditto; .">, (Jruy commissure: '>. 

 White commissure; 7, 7, Superior grey cor- 

 nua ; 8, 8, Inferior grey cornua ; 9, Central 

 canal. 



These bands are named the 

 trhltf' and ;/'"''/ commissures of the 

 spinal cord (Fig. 318). 



Notwithstanding the presence 

 of these two commissures between 

 the lateral halves of the spinal 

 axis, these latter do not the less con- 

 stitute two symmetrical systems, 

 whose structure will now be 

 studied. 



Each medullary cord repre- 

 sents a semi-cylinder of white sub- 

 stance, in the centre of which is 



a moss of grey matter, that varies somewhat in quantity in different regions, 

 but the arrangement of which is everywhere the same. Thus, inwardly, this 

 grey matter joins the grey commissure ; above, it sends off a thin prolongation 

 which traverses the thickness of the medullary cord (superior grey cornu >, 

 to reach the bottom of the superior collateral fissure; below, it 



to an analogous, though a thicker and a more irregular, prolongation 

 ( inferior grey coruu), which is directed well in front of the inferior roots, 

 but does not reach the surface of the cord. In consequence of this arrange- 

 ment, the grey substance of the medulla forms altogether a kind of capital 

 II, whoso horizontal branch is perforated in the middle by the central 

 canal. 



This disposition of the grey substance causes the white matter to bo 

 divided, in each lateral moiety of the spinal axis, into throe cords or 

 secondary columns ; the super'wr of these is perfectly isolated, and is coni- 

 i n the middle superior fissure and the origin of the sensitive 



ka ; another, the inferior, united to that of the opposite side by the white 

 commissure, is limited, inwardly, by the inferior median fissure, and out- 

 wardly by the lino of origin of the motor nerve-roots; while a third, the 

 lutmif or in termed iale, thicker than the others, is confounded superficially 

 with the inferior, and formed by all that portion of the medulla situated 



Iveen the lines of origin of the superior and inferior roots. Of these 

 three columns of the medullary axis, the first is sensitive ; the other two, 

 which in reality are only one. are nu-t -r. 

 STRUCTURE. Independently of tbe epithelium mentioned when describing 



