496 THE SPINAL CORD. 



The grey substance, as seen in a transyerse section of any part of 

 the cord, presents two crescent-shaped masses, placed one in each lateral 

 half, with their conA'exities towards one another, and joined across the 

 middle by a transverse portion, the g^-eij or posferior commissure of the 

 cord. Each of these grey crescents has an anterior and ^^wsterior cornu or 

 horn. The posterior, generally longer and narrower, approaches tlie 

 posterior lateral fissure : the anterior, shorter and thicker, extends 

 towards the place of attachment of the anterior roots of the nerves, 

 thus dividing the antero- lateral column into the anterior and lateral 

 portions of white substance. On the outer surface of each crescent, be- 

 tween the two cornua, processes from the grey substance form a network 

 which encloses portions of the white column. This is seen especially in 

 the cervical region, A layer of white substance separates the grey 

 commissure from the bottom of the anterior median fissure, and is 

 named the anterior or white commissure. 



The back part of the posterior horn, which is somewhat enlarged, is 

 called the caput cornu postcrioris (fig. 350, a a) ; the narrower poi'tion 

 which connects it with the rest of the grey substance being called 

 the cervix cornu (fig. 350, l). At the tip of the caput cornu the grey 

 matter has a peculiar semitransparent aspect, whence it was named 

 by Eolando substantia cincrea gelatinosa. 



The grey crescents vary in form in different parts of the cord (see 

 fig. 348). In the dorsal region both anterior and posterior cornua are 

 narrow. In the'cervical and lumbar regions the anterior cornua are large 

 and broad, constituting cylindrical or prismatic columns of grey sub- 

 stance. The posterior cornua are narrow in the cervical, but very broad 

 in the lumbar region. The grey matter is seen in a series of sections 

 to be, relatively to the white, most abundant in the lumbar region of 

 the cord, less so in the cervical region, and least so in the dorsal. 

 The actual amount of white matter is greatest in the neck, of grey 

 matter in the lumbar region. Towards the lower end of the cord the 

 double crescentic form gradually disappears, and the grey matter is 

 collected into a central mass, which is indented before and behind and 

 at the sides. At its extreme point, according to Eemak and Valentin, 

 the cord consists of grey matter only. 



Fig. 349. Fig. 349. — Section of Lower Extremity op Spinal Cord. Magnified 



ABOUT Six Times. 



__^ In A, the peculiar form of the lower extremity of the central canal is 



seen, and in B, its opening on the posterior surface. 



The commissure connecting the two halves of the cord 

 measures -:?^i\i or ^-^^th of an inch in average thickness, and 

 consists, as already mentioned, of two portions — anterior 

 white, and posterior grey commissures. The anterior is 

 proportional in size to the size of the nerve roots. The pos- 

 terior is largest in the upper part of the conns medullaris. 

 In it is contained the centi-al canal. 



Central canal. — Extending through the whole length of the spinal 

 cord, in the substance of the grey commissure, there is a minute 

 central canal which, in prepared transverse sections of the cord, is 

 barely visible, as a speck, with the naked eye. Superiorly, it is con- 

 tinued into and opens out at the calamus scriptorius of the fourth 



