CH. XLV1II.] THE CONVOLUTIONS OF THE CEREBRUM 



691 



In an early embryonic stage of the human foetus the brain is also 

 smooth, but as development progresses the sulci appear, until the 

 climax is reached in the brain of the adult. 



The sulci, which make their appearance first, both in the animal 



FIG. 427. 



A. Cerebral Hemisphere of adult Maeacque monkey. 



B. Cerebral Hemisphere of child shortly before birth. 



The two brains are very much alike, but the growth forwards of the frontal lobes even at this early 

 stage of development of the human brain is quite well seen. S. fissure of Sylvius ; B, fissure of 

 Rolando. 



scale and in the development of the human foetus, are the same. 

 They remain in the adult as the deepest and best-marked sulci; they 

 are called the primary fissures or sulci, and they divide the brain into 



Fio. 428. Brain of the Orang, jj 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 oblongata. As contrasted with the human brain, the 

 frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the teraporo-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. 



lobes; the remaining sulci, called the secondary fissures or sulci, 

 further subdivide each lobe into convolutions or gyri. 



A first glance at an adult human brain reveals what appears to 

 be a hopeless puzzle ; this, however, is reduced to order when one 



