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NERVOUS CENTRES. (Human Anatomy. 
Fig. 394. 
Tue Encepuaton.) 
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of the rabbit, the beaver, the guinea-pig, the 
agouti shew these fissures. They are generally 
‘regular in different individuals of the same 
_ genus, and they are symmetrical, i.e., of the 
same length and direction, and occupy the 
‘same place on each hemisphere. 
Leuret remarks, in reference to the dogma 
of Gall and Spurzheim, that the presence and 
number of the convolutions are in direct rela- 
_ tion to the volume of the brain, that such is far 
from being universally the case; and I am 
glad to refer to so excellent an authority in 
confirmation of the view which I have advo- 
cated respecting the true signification of the 
cerebral convolutions. According to this ana- 
tomist, the ferret, which has several well-marked 
convolutions on each hemisphere, has a brain 
no larger than that of the squirrel, which is 
_ entirely devoid of them, and which has not 
even the few fissures which faintly indicate 
their first developement in the brains of the 
rabbit, the beaver, the agouti, &c. And these 
animals last named have the brain actually 
larger than that of the cat, the pole-cat, the 
roussette, ( Pteropus vulgaris, ) the unau, ( Bra- 
. dypus didactylus, ) the sloth, ( Bradypus tridac- 
tylus, 
) and the pangolin, all of which possess 
convolutions. 
All mammiferous animals, excepting those 
mentioned in the preceding paragraphs, have 
convolutions which exhibit more or less of 
complication. This complication has evidently 
no connection with the general organization of 
the animal, inasmuch as we find animals, in 
the same family with those which possess 
numerous convolutions, exhibiting a very slight 
developement of them. The monkeys, the 
dolphin, the elephant, exhibit the most nume- 
rous convolutions of any of the Mammalia 
inferior to man, in whose brain the convoluted 
surface reaches its highest point. 
Each fold on the surface of the brain is 
ordinarily called a convolution, whatever be 
it$ position, size, or direction. It consists of 
a fold of grev matter, enclosing a process of 
Superior surface of the right hemisphere of the adult human brain. 
j 
‘The undulating form of many of the convolutions is very well seen, and the general characters of the 
‘ convoluted surface are displayed. 
white or fibrous matter. On each side of it is 
a sulcus or groove, in which we find the same 
elements, a fold of grey or vesicular matter— 
concave externally, convex internally — the 
fibrous matter adhering to its convex surface. 
As the convolution exhibits no essential diffe- 
rence of structure from the sulcus, it is plain 
that the former portion of the brain’s surface 
cannot differ in physiological office from the 
latter. We describe particular convolutions, 
not because they are to be regarded as 
endowed with special functions distinct from 
the less prominent portions of the cerebral 
surface, the sulci, which are continuous and 
identical in structure with them, but because 
they afford good indications of a particular 
arrangement of the surface of the hemispheres 
by which one brain may be coveniently com- 
pared with another, whether they belong to the 
same or to different groups of animals. 
The folded arrangement of the surface of 
the hemispheres, dependent as it is upon the 
grey matter, is evidently destined to bring the 
central and deep-seated parts of the hemispheres 
into union with a large extent of vesicular 
surface. 
That the disposition of the convolutions, 
like that of all other parts of animal bodies, 
follows a particular law, is well illustrated by 
comparing the brains of different groups of 
animals, in their gradation from the more simple 
to the more compiex. 
M. Leuret very justly makes a distinction 
between those convolutions which are constant, 
and to be found throughout the whole series of 
convoluted brains, occupying the same position, 
and differing only in size and extent of connec- 
tions, and those which are not constant, even 
in the brains of the same group of animals, 
but are dependent on the extent of the primary 
ones, and the connections which they form 
with others near them. According to this idea 
we may classify the convolutions as primary 
and secondary. 
The primary convolutions are all formed after 
