THE SPINAL CORD. 



the two median fissui-es mentioned in describing the exterior of the cord 

 the inferior of which is wider and deeper than that of the superior, whose 

 situation is scarcely perceptible. 



before the other, and do not meet 



Fig. 318. 



These two fissures advance one 

 so as_ to completely divide the 

 cord into two lateral halves, but 

 remain separated by two tliin 

 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 

 fissure. 



These bands are named the 

 white and grey commissures of the 

 spinal cord (Fig. 318). 



Notwithstanding the presence 

 of these two commissures between 

 the lateral halves of the spinal section op the spinal cord of the horse 

 axis, these latter do not the less con- bLmeters™'^" ^^'''°'' '' "^'*'''"^" ^'"' 

 Stitute two symmetrical systems, i, Superior median fissure; 2, Inferior median 

 whose structure will now be fissure; 3, 3, Superior collateral fissures; 

 studied. 4, 4, Inferior ditto ; 5, Grey commissure ; 6, 



Each medullary cord re pre- ^'^''^ commissure; 7, 7, Superior grey cor- 



„„„. • T 1*^ » •■., '^i nua; 8, 8, Inferior e;rey cornua; 9, Central 



sents a semi-cylmder of white sub- ^^^^^^ ' ' ^ •' ' ' 



stance, in the centre of which is 



a mass 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 gives 

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

 (inferior grey cornu), 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 

 H, whose horizontal branch is perforated in the middle by the central 

 canal. 



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

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

 secondary columns ; the superior of these is perfectly isolated, and is com- 

 prised between the middle superior fissure and the origin of the sensitive 

 roots ; 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 line of origin of the motor nerve-roots ; while a third, the 

 ■lateral or intermediate, thicker than the others, is confounded superficially 

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

 between 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 motor. 



Stkuotuee. — Independently of the epithelium mentioned when describing 



