THi: SPINAL CORD. 



757 



Internal Conformation and Structure of the Spinal Cord. 



In making a transverse section of any portion of the cord, we may con\-ince 

 ourselves that it has an internal cavity. This central canal is elliptical, and 

 hned by a simple layer of columnar ciliated epithelium, resting on a thin con- 

 nective membrane — the fpendf/triis of Virchow. (The cilia of these cells are only 

 to be seen in early life ; in advanced age the canal may be closed, the cells then 

 becoming much altered in form from pressure. The canal is immediately sur- 

 rounded by spongy horny matter.) This section also shows the two median 

 fissures mentioned in describing the exterior of the cord, the inferior of which is 

 wider and deeper than the superior, the situation of the latter being scarcely 

 perceptible. 



These two fissures lie one before the other, and do not meet so as to com- 

 pletely divide the cord into two lateral halves, but remain separated by two 

 thin horizontal and superposed bands 

 of nerve matter, that pass from one 

 end to the other of the medullary axis. 

 The inferior, formed of white sub- 

 stance, corresponds to the bottom of 

 the inferior fissure : while the superior, 

 composed of grey matter, meets the 

 superior fissure. 



These bands are named the irhite 

 and grey commissures of the spinal cord 

 (Figs. 418,419, 420). 



Notwithstanding the presence of 

 these two connnissiu'es between the 

 lateral halves of the spinal axis, these 

 latter do not the less constitute two 

 symmetrical systems, the structure of 

 which will now be studied. 



Each medullary cord represents a 

 semi-cylinder of white substance, 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 that traverses the substance of the medullary cord (superior grey 

 cormi), 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 arrangement, the grey 

 substance of the cord forms altogether a kind of capital H, the horizontal 

 branch of which is perforated in the middle by the central canal. 



This an'angement 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 comprised between 

 the middle superior fissure and the origin of the sensitive roots ; another, the 

 inferior, united to that of the opposite side Toy the white commissure, is limited, 

 inwardly, by the inferior median fissure, and outwardly by the hne of origin of 



SECTION OF THE SPINAL CORD OF THE HORSE 

 AT THE LUMBAR REGION. MAGNIFIED TWO 

 DIAMETERS. 



1, Superior median fissure ; 2, inferior median 

 fissure; 3,3, superior collateral fissures; 4, 4, 

 inferior ditto ; 5, grey commissure ; 6, white 

 commissure , 7, 7, superior grey cornua ; 8, 8, 

 inferior grey cornua ; 9, central canal. 



