694 
surfaces were equal, neither of them would be 
thrown into folds. In examining thesurfacecalled 
centrum ovale, which is exposed by a horizontal 
section through the hemisphere above the level 
of the corpus callosum, we obtain an explanation 
of the formation of the convoluted surface of the 
brain. That plane of fibrous matter is surrounded 
by an undulating margin of vesicular matter, 
the foldings of which give rise to the convoluted 
appearance of the cerebral surface. The fibrous 
matter is adapted to this irregular surface, not 
by any similar folding, but by the prolongation 
of its fibres into the concavities of the folds. 
It is only by means of these Prvlongenons that 
an equality obtains between the surface of grey 
matter and that of fibrous matter which it 
covers. In brains devoid of convolutions, 
the vesicular and fibrous surfaces are applied 
to each other as two layers disposed in con- 
centric circles. There are no irregularities in 
either one or the other. But any increase in 
the extent of the grey surface involves a cor- 
responding complication in that of the fibrous 
matter, which is effected by the prolongation 
of the fibres at certain situations. Were we to 
suppose two brains in which the quantity of 
fibrous matter in the hemispheres was equal, 
the quantity of grey matter in one might be 
increased considerably, and therefore become 
convoluted without involving any other altera- 
tion in the fibrous matter than the elongation 
ig. 393. 
Vertical section of the adult human brain, (After Arnold.) 
NERVOUS SYSTEM. (Nervous Cextazs. Tur Encepnaton.) 
of certain bundles of fibres at particular — 
situations. . ae 
The existence of convolutions on the surface 
of the hemispheres, as contrasted with the ab- 
sence of them, indicates an increase in the de- 
velopement of the dynamic matter. A convo 
luted brain, even although ge malle 
than one with a smooth surface, would yet in- 
dicate a higher degree of mental power, im 
much as it possesses a larger qastag 
vesicular matter relatively to its fibrous matter, 
Cerebral convolutions are wanting in all th 
classes below Mammalia. They are likewis 
absent from the brains of many animals of | 
families Rodentia, Cheiroptera, Insective 
some of the Marsupialia, and Monotrem: 
The brains of these Mammalia resemble ve 
closely, as regards the characters of the cereb 
hemispheres, the brain of Birds. There is n 
a trace of a convolution Op them, and 
only fissure is the imperfectly developed one 
Sylvius. The squirrel, the bat, the mole a: fo 
examples of brains deficient in convolution 
In some genera of the families Insectivora 
Marsupialia, however, we find an approach 
the convoluted cerebral surface in a 
pressions marked on the exterior of each | 
misphere. The fissure of Sylvius is more ¢ 
veloped, and certain depressions, taking for t 
most part a longitudinal course, are seer 
the surface of each hemisphere. The br 
Ven 
Certain | 
The position of the internal convolution with reference to the corpus callosum is well display d 
median lobe of the cerebellum has been cut through, and the fourth ventricle exposed. a, a 
convolution, (d’ourlet, Foville); c, corpus callosum; 0, fornix; n, septum lucidum; f, 
i, anterior commissure, h, hypophysis, or pituitary body; ¢, pons Varolii; II, second pait ms 
nerves; IV, fourth ventricle. 
