692 



STRUCTURE OF THE CEREBRUM 



[CH. XLV1II. 



studies the brain in different stages of development, or compares the 

 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 (fig. 427), 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. 428 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. 



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 Eeil. 



2. The fissure of Rolando (the central fissure) runs from about 



R O N TA J. 



FIG. 429. Eight cerebral hemisphere, outer surface. 



the middle of the top of the diagram (fig. 429), downwards and 

 forwards. 



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

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

 it is longer (see fig. 428), as it is not interrupted by annectent gyri. 



These three fissures divide the brain into five lobes : 



1. The frontal lobe, ; in front of the fissure of Eolando. 



2. The parietal lobe; between the fissure of Eolando and the 

 external parieto-occipital fissure. 



3. The occipital lobe ; behind the external parieto-occipital fissure. 



4. The temporo-sphenoidal lobe ; below the fissure of Sylvius. 



5. The Island of Eeil. 



It will be noticed that the names of the lobes correspond to those 

 of the bones of the cranial vault which cover them. There is no 



