694 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



surfaces were equal, neither of them would be 

 thrown into folds. Inexaminingthesurfacecalled 

 centrum ovule, 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 prolongations 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 



of certain bundles of fibres at particular 

 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 dynamic 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 below 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 regards the characters of the cerebral 

 hemispheres, the brain 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 brains deficient in convolutions. 

 In some genera of the families Insectivora 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 



Fis. 393. 



Vertical section of the adult human brain. (After ArnuhL) 



The position of the internal convolution with reference to the corpus callosum is well displayed. The 

 median lobe of the cerebellum has been cut through, and the fourth ventricle exposed, a, a, a, internal 

 convolution, (d'ourlet, Foville) ; c, corpus callosum ; o, fornix ; n, septum lucidum j /, pineal body ; 

 i, anterior commissure ; h, hypophysis, or pituitary body : t, pous Varolii ; II, second pair or optic 

 nerves ; IV, fourth ventricle. 



