CH. XLVL] 



THE CEREBRAL CONVOLUTIONS 



663 



brain of man with that of the lower animals. The monkey's brain 

 in particular has given the key to the puzzle, because there the 

 primary fissures are not obscured by the complexity and contorted 

 arrangement of secondary fissures. 



The preceding figure, comparing the brain of one of the lower 

 monkeys with that of the child shortly before birth, shows the close 

 family likeness in the two cases. 



Fig. 484 gives a representation of the brain of one of the higher 

 monkeys, the orang-outang, where there is an intermediate condition 

 of complexity by which we are led lastly to the human brain. 



FIG, 484. Brain of the Orang, 3 natural size, showing the arrangement of the convolutions. Sy, fissure 

 of Sylvius; R, fissure of Rolando; EP, external parieto-occipital fissure; Olf, olfactory lobe; Cb, 

 cerebellum; PV, pons Varolii; MO, medulla oblougata. As contrasted with the human brain, the 

 frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the temporo-sphenoidal 

 lobe very prominent, and the external parieto-occipital fissure very well marked. Note also the 

 bend or genu in the Rolandic fissure. This is found in all anthropoid apes. 



Let us take first the outer surface of the human hemisphere ; the 

 primary fissures are 



1. The fissure of Sylvius ; this divides into two limbs, the posterior 

 of which is the larger, and runs backwards and upwards, and the 

 anterior limb, which, passing into the substance of the hemisphere, 

 forms the Island of Ecil. 



2. The fissure of Rolando, running from about the middle of the 

 top of the diagram (fig. 485) downwards and forwards. 



3. The external parieto-occipital fissure (PAR. oc. F) parallel to the 

 fissure of Eolando but more posterior and much shorter ; in monkeys 

 it is longer (see fig. 484). 



These three fissures divide the brain into five lobes : 

 \. The frontal lobe', in front of the fissure of Bolando. 



