380 
POPULAR SCIENCE REVIEW. 
If a chick be examined in a hen’s egg which has been allowed to 
hatch for twelve hours, or if the embryo of any vertebrate animal 
be examined at a similarly early period, it will be seen to exhibit 
a long open furrow, the walls of which are the first portions of 
the animal to be formed. The most superficial layer of sub- 
stance entering into the construction of this furrow may be 
described as a long ribbon, consisting of two symmetrical parts 
separated by a longitudinal groove : this is the embryo brain and 
spinal cord, constituting one continuous structure, the cerebro- 
spinal axis. The parts which support the ribbon form in like 
manner the cranium and the spinal canal, primarily undistin- 
guishable one from the other. The edges of the furrow rise up 
and become united, so that the open furrow is converted into a 
closed cylinder ; and similarly the ribbon within it has its lateral 
edges brought together, so that the brain and spinal cord, at an 
early period of their development, form one continuous tube. 
The walls of the tube so formed become ultimately much 
thickened and exhibit two kinds of texture, which, from their 
colour, are distinguished as the grey and the white. In the 
case of so much of the tube as lies in the spinal canal and is 
afterwards termed spinal cord, the development proceeds very 
regularly; white matter is deposited on the outer wall of the 
cylinder, and gre}^ matter on the inner wall, until it appears solid. 
A minute canal, however, the central canal of the spinal cord, con- 
tinues to traverse its whole extent throughout life, and is the re- 
mains of the original hollow of the tube. Towards the lower part 
of the cord in birds there is even a space called the sinus rhom- 
boidalis, where the cylinder is never completed, and the central 
canal is open on the dorsal aspect. Now, however different the 
brain may be in the adult condition from the spinal cord, it is 
extremely interesting to note that it is the anterior portion of 
the same cylinder, but that the cylinder undergoes some bend- 
ings, its walls are greatly thickened in some places and imperfect 
in others, and the continuation of the central canal is in some 
places greatly dilated, and in others contracted. 
As respects texture, there is much in common between the 
brain and spinal cord. They are similar in appearance, and 
both consist of true nerve tissues, with a fine reticulum of sup- 
])orting substance in which those more important elements are 
imbedded. The proper nerve tissues are two in number, nerve 
fibres and nerve corpuscles : the nerve fibres are long threads 
which have the property of transmitting along their course a 
certain change of condition which constitutes nervous influence, 
and which, it may l)e mentioned, is a purely physical action, not 
electrical, hut involving in its operation electrical changes. 
Nerve fibres transmit this iiifluerjce, hut have no power of 
originating, directing, or modifying it : they are simply con- 
