146 PHILOSOPHY OF ZOOLOGY. 
enveloped by the same integuments which we have already 
noticed as belonging to the brain itself. It differs from the 
brain, however, in the arrangement of its component parts, 
the medullary matter here occupying the surface, while the 
cineritious is disposed towards the centre in a cross-like 
form. It is divided longitudinally ito two equal halves, 
by a channel which is very obvious on the dorsal aspect, 
and on each side there is likewise a groove. By some ana- 
tomists, it is considered as consisting of two medullary cords 
only, divided in the direction of the mesial line; while 
others consider it as consisting of four cords, two anterior 
and two posterior. Considerable support seems to be af- 
forded to the last opinion, by the circumstance of its origi- 
nating from four medullary cords, two from the cerebrum, 
and two from the cerebellum. The two former may be 
considered as formmg the anterior, and the two latter the 
posterior portions. These cords, however, are brought in- 
to intimate union by numerous filaments, which pass from 
the one to the ether. Mr Szwe xt, of the Vetermary Col- 
lege, London, has ascertained the existence of a canal in 
the centre of the spinal marrow in the horse, bullock, sheep, 
hog, and dog. It extends, uninterruptedly, from the cala- 
mus scriptorius to the cauda equina; is lined by a mem- 
brane, resembling the tunica arachnoidea, and contains a 
transparent colourless fluid, like that which is contained in 
the ventricles of the brain *. 
3. The nerves may be regarded either as originating 
from, or terminating in, the brain or spinal marrow. We 
shall consider them, for the present, in the former point of 
view. ‘The integuments of the nerves resemble, in appear- 
ance, those of the brain, closely invest them on all sides, and 
* 6 A Letter on a Canal in the Medulla Spinalis of some Quadrupeds.”— 
Phil. Trans, 1809, p. 146. 
