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POPULAR SCIENCE REVIEW. 
survey of the spinal cord, the simplest portion of the cerebro- 
spinal axis, in order that the general reader may form some 
conception of the kind of mechanism which extends through 
the more obscure and intricate portion, the brain. To explain 
fully the extremely complex structure of the brain would require 
much greater detail than is allowable in an article like this, but 
a general idea of the most important facts will best be arrived 
at by pursuing the account of its early development, which we 
have already begun. 
The cylinder which we have traced in the embryo, so far as 
the spinal cord is concerned, is, immediately on its closure, 
expanded in its cranial part into a series of three primordial 
vesicles, and immediately afterwards two little hollow buds, 
called the hemisphere vesicles, project laterally from the fore- 
most of the series. Without tracing the history of the primordial 
vesicles, it is sufficient for our present purpose to point out that 
the cerebellum is originally a part of the hindermost, projecting 
upwards as a hollow pouch, and that it is quite certain, from 
experiments on the lower animals, that no consciousness what- 
ever resides in any of the parts developed from that vesicle ; 
also it is equally certain that not more than the very feeblest 
consciousness resides in those parts into which the walls of 
the two other primordial vesicles are developed. These parts 
are devoted to the carrying on of obscure functions connected 
with the sensibility and movements of the body strictly com- 
parable with the functions of the spinal cord, and entirely of a 
j)hysical description ; the organs of the mental faculties are the 
developed hemisphere vesicles, and these only. The hemisphere 
vesicles rapidly enlarge and extend backwards over and around 
the other parts of the brain, so as to reach to the cerebellum 
behind, come in contact with the whole roof and sides of the 
skull and a hirge part of its floor, and press one against the 
other in the middle line of the whole length of the skull for an 
average depth of a couple of inches; and early in embryonic 
life they are already much the most bulky parts of the brain. 
'I'he grey matter which lines the whole length of the cerebro- 
spinal cylinder fails to be developed in the hemisphere vesicles, 
except at one part placed at the neck of the vesicle, and called 
by anatomists the corpus striatum, but of which we know nothing 
in respect of function, and can only note that it is traversed by 
the whole mass of fibres joining the hemisphere vesicles with 
the cord and cerebellum. The whole of the rest of the hemi- 
sfdiere vesicle, or, as it is termed, the cerebral hemisphere, 
consists of an enormous mass of white matter, with a superadded 
layer of grey matter on the outside. The cerebellum has the 
same peculiarity of having its grey matter on the surface, and 
it is curious to note that both the grey matter on the cerebellum 
