THE SPINAL COED. 



669 



the two median fissures 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. 



These two fissures advance one before the other, and do not meet 



Fig. 318. 



so as to completely divide the 



cord into two lateral halves, but 



remain 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 



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 



axis, these latter do not the less con- DIAMETERS. 



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



whose Structure will now be fisstire; 3, 3, Superior collateral fissures; 

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



Eaoh medullarv cord renre- White commissure; 7, 7, Superior grey cor- 

 mecmiiaiy Cora repre Inferior grey comua; 9, Central 



sents a semi-cylinder of white sub- cana \ 

 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. 



STEUCTUKE. Independently of the epithelium mentioned when describing 



