280 INNERVATION. [CHAP. X. 



of the convolutions, some distance beyond those which penetrate 

 the gray matter of the sulci, the fibrous matter is adapted to it. 



The existence of convolutions on the surface of the hemispheres 

 affords evidence of a large relative amount of the dynamic or vesicu- 

 lar nervous matter in those segments of the brain, and their number 

 and complexity are a measure of the extent to which the vesicular 

 surface is increased. Of two brains, equal as regards bulk, and occu- 

 pying the same space, that which has the more numerous convolu- 

 tions on its surface has the greater quantity of vesicular matter, 

 and must be regarded as physiologically the more potential. 



A remarkable gradation is observable as regards the number of 

 the cerebral convolutions from the lowest mammalia up to man. 

 Some of the Rodentia, Cheiroptera, and Insectivora, occupy the 

 lowest place; and monkeys, the elephant, and the whale, rank 

 next to Man, in whom the convolutions reach their highest point 

 of development. In the rat, mole, &c., the surface of the brain is 

 perfectly smooth : and the only tendency to complication which it 

 exhibits is to be found in the convolution of the gray matter at the 

 fissure of Sylvius. The brains of these animals resemble, in this 

 respect, to a striking degree, those of birds, which are equally des- 

 titute of all semblance of a convolution. But in the rabbit, guinea- 

 pig, beaver, &c., the occurrence of certain fissures on the surface of 

 the hemispheres, and the greater depth of the fissure of Sylvius, 

 denote the first steps in the development of convolutions. 



A further stage of development is indicated by the existence of 

 certain rounded folds which generally take a direct course parallel 

 to the long axis of the hemispheres. These folds are but few in 

 number, and quite simple, but may be readily distinguished from 

 the rest of the cerebral surface, by the fissures which bound them. 



However complicated and numerous the convolutions of the most 

 highly developed brains may be, it cannot be supposed that their 

 arrangement is accidental, or has reference merely to the space 

 within which the brain is enclosed. On the contrary, there seems 

 no doubt that the position, size, and connexion of certain primary 

 folds influence mainly the number and variety of those which 

 occupy the intervening spaces. This interesting point has been 

 strongly insisted upon by M. Leuret, who shews, by comparison 

 of the most completely convoluted brain with those in which the 

 folds are few and simple, that the convolutions of the latter, which 

 are, as it were, the original landmarks in this intricate arrange- 

 ment of the cerebral surface, may be demonstrated in each succes- 

 sive group of brains which form a stage in the ascending series. 



