500 



COMPARATIVE ANATOMY 



was, however, out of all proportion to the enlargement of the rest of the 

 brain. This increase was made possible by the complex folding of the 

 outer layers of the brain and resulted in the formation of the gyri and 

 sulci which are such a characteristic feature of the surface of the human 

 brain. (Figs. 418, 419) 



rRlM'NEOPALUI 



"\ AREA 



PVRIFOftMiS 



CLAUSTOUM 



T STRiATt AKteltf 



Tu^Elt. OUF. 



Fig. 415. — Diagrams of transverse sections of the right cerebral hemisphere. To 

 indicate how the primitive reptiHan arrangement (left; above) becomes transformed in 

 the evolution of the mammal, and especially the way in which the hypopallium of the 

 reptile {H. i, 2, 3, 4, 5) becomes converted into the upper part of the nucleus caudatus 

 {N.C), putamen (F) of the nucleus lentiformis, and claustrum. (From Wilder's 

 "History of the Human Body," after G. Elliot Smith; courtesy of Henry Holt and 

 Company.) 



Hemispheres in Man. The human cerebral hemispheres are sepa- 

 rated in the median plane by a longitudinal fissure. Viewed from the 

 surface, four topographic but not functional divisions are recognized, 

 the frontal, parietal, occipital, and temporal lobes. Each is subdivided 

 by sulci or fissures into convolutions or gyri. The gyri of the frontal 

 lobe are the superior, middle, inferior, rectus, and orbital. Those of the 



