CEREKRAL CONVOLUTIONS. 



635 



together, viz., the marginal convolution of the median fissure and that of the Sylvian 

 fissure, and occupy the outer or convex surface of the cerebral hemisphere. 



Leuret, by an extended comparison of the brains of different animals, was led to 

 divide mammals into fourteen groups, according to the disposition of the convolutions. 



In the lowest or simplest group, including the bat, mole, and rat, the Sylvian 

 fissure is the only division of the surface present, or along with it a few very slight 

 sulci. In a higher group, containing the fox and dog, and presenting in a marked 

 form the t} r pical mode of division, Leuret recognises as fundamental six convolutions 

 four external, including the superior marginal and that of the fissure of Sylvius, 

 and two internal, viz., the supraorbital and gyrus fornicatus. In other groups, 

 together with various other modifications of form by subdivision or by union through 

 supplemental ones, the number of the fundamental convolutions is frequently reduced 

 to five or to four. 



In the brain of the elephant, on the other hand, placed by Leuret in the thirteenth 

 group, he recognises the superior transverse convolutions; and in the last group, 

 comprehending the quadrumana, these transverse convolutions are two in number, 

 and are separated by the groove, named by Leuret fissure of Rolando. These trans- 

 verse or ascending parietal convolutions are a constant and well-marked feature of the 

 human bi'ain, in which they attain their highest development. 



Fi-. 366. 



Fig. 366. OUTLINE OP THE INNER SURFACE OP THE EIGHT HALF OP THE BRAIN, 



SHOWING THE PRINCIPAL LOBES AND CONVOLUTIONS ACCORDING TO GrRATIOLET. 



F, frontal lobe ; P, parietal ; 0, occipital ; T, temporal ; r, fissure of Rolando ; /, 

 fronto-parietal fissure ; p', inner perpendicular or occipito -parietal fissure ; h, the calca- 

 rine fissure of Huxley, and with the line continued forwards between g and h', the hippo- 

 campal fissure ; ti, convolution of the hippocampus ; g, gyrus fornicatus or convolution 

 of the corpus callosum ; s, Sylvian fissure ; I, olfactory bulb ; II, optic nerve ; III, third 

 nerve; C, cerebellum. 



More recently, Gratiolet has arranged the convolutions with great detail, according 

 to their most distinguishing common features in man and the simiae. On the 

 external surface of the hemisphere he distinguishes five lobes, viz., the frontal and 

 parietal above the fissure of Sylvius ; the temporo-splienoidal below that fissure ; the 

 occipital behind it, and the island of Reil, or central lobe, within the fissure. The 

 frontal lobe he divides into an orbital and a frontal portion, and in the frontal portion 



