


THE LINGERING ADMIRERS OF PHRENOLOGY. 589 
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 color, are distinguished as the gray 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 devel- 
opment proceeds very regularly ; white matter is deposited 
on the outer wall of the cylinder, and gray matter on the 
inner wall, until it appears solid. A minute canal, however, 
the central canal of the spinal cord, continues to traverse its 
whole extent throughout life, and is the remains of the orig- 
inal hollow of the tube. Towards the lower part of the cord 
in birds there is even a space called the sinus rhomboidalis, 
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 por- 
tion of the same cylinder, but that the cylinder undergoes 
some bendings, 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 
supporting substance in which those more important ele- 
ments are embedded. 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 con- 
stitutes nervous influence, and which, it may be mentioned, 
is a purely physical action, not electrical, but involving in its 
operation electrical changes. Nerve fibres transmit this 
influence, but have no power of originating, directing, or 
modifying it: they are simply conductors, and such nerve 
fibres are the essential elements in all the nerves throughout 
