SPINAL CORD—CRANIAL NERVES. 489 
Spinal cord.—After the formation of the brain vesicles, 
the remainder of the medullary canal forms the spinal 
cord. 
The canal is for a time continuous posteriorly with the 
food canal beneath, so that a >-shaped tube results. The 
connection between them is called the neurenteric canal 
(Fig. 254, 7e.c.), and though it is only temporary, its frequent 
occurrence is of much interest. 
The wall of the medullary canal becomes very much 
thickened, the roof and floor grow less rapidly, and thus 
the cord is marked by ventral and dorsal longitudinal 
furrows. At the same time, the.canal itself is constricted, 
and persists in the fully-formed structure only as a minute 
canal lined by ciliated epithelium, and continuous with the 
cavity of the brain. * 
In the cord it is usually easy to distinguish an external region of 
white matter, composed of medullated nerve-fibres, and an internal 
region of grey matter, containing ganglionic cells and non-medullated 
fibres. 
The arrangement of the grey matter, together with the longitudinal 
fissures, give the cord a distinct bilateral symmetry, which is sometimes 
obvious at a very early stage. 
The brain substance is also composed of grey and white matter, but 
there, at any rate in higher forms, the arrangement is very complicated, 
Cranial nerves. — The origin and distribution of the 
cranial nerves may be summarised as follows :— 
[TaBLE, 
