LIMIT CUI1VOIlltell 1111S can only be well displayed by strip ping off the pia mater. The ap pearance which is then presented has been variously described by different writers. It has always seemed to me to resemble the folded surface formed by the mucous mem brane of the stomach when the mus cular coat is very much contracted. The rugm of that membrane be come enormously developed by the excessive contraction of the mus cular coat : the mucous membrane not possessing any contractile power is thrown into thin folds to adapt it to the diminished capacity of the stomach. Its folded state indicates a great disproportion between the extent of the mucous surface and that of the muscular tunic. If both surfaces were equal, neither of them would be thrown into folds. In examin ingthe surfaceealled centrum ovule, which is exposed by a horizontal section through the hemisphere above the level °idle corpus callosum, we obtain an explanation of the formation of the convoluted surface of the brain. That plane of fibrous matter issurrounded by an undulating margin of vesicular matter, the foldings of which give rise to the convoluted appearance of the cerebral surface. The fibrous inatter 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 prolongations that an equality obtains between the surface of grey rnatter 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, vvhich 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 elontsation of certain bundles of fibres at partkular situations.
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 dynainic matter. A convo
luted brain, even although actually smaller than one with a smooth surface, would yet in dicate a higher degree of mental power, inas much as it possesses a larger quantity of the vesicular matter relatively to its fibrous matter.
Cerebral convolutions are wanting in all the classes belovr Mammalia. They are likewise absent from the brains of many animals of the families Rodentia, Cheiroptera, Insectivora, some of the Marsupialia, and Monotremata. The brains of these Mammalia resemble very closely, as reg,ards the characters of the cerebral hemispheres, the bmin of Birds. There is not a trace of a convolution upon them, and the only fissure is the imperfectly developed one of Sylvius. The squirrel, the bat, the mole afford examples of bmins deficient in convolutions. In some genera of the families Insectivom and Marsupialia, however, we find an approach to the convoluted cerebral surface in certain de pressions marked on the exterior of each he misphere. The fissure of Sylvius is more de veloped, and certain depressions, taking for the most part a longitudinal course, are seen on the surface of each hemisphere. The brains 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 saine 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 development 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 dy pus didaetylus,) the sloth, ( Bradypus tridac tylus,) and the pangolin, all of which possess convolutions.