CH. XLVI.] CEREBRAL CONVOLUTIONS. 653 



of the surface that enables a very large amount of the precious 

 material called the grey matter of the cortex to be packed within 

 the narrow compass of the cranium. In the lowest vertebrates 

 the surface of the brain is smooth, but going higher in the animal 

 scale the fissures make their appearance, reaching their greatest 

 degree of complexity in the higher apes and in man. 



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 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 lobes ; the remaining sulci, called the secondary 

 fissures or sulci, further subdivide each lobe into convolutions -or gyri. 



B 



Fig. 485. 



A. Brain of adult Macacque monkey. 



B. Brain 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. 8, fissure of 

 Sylvius ; B, fissure of Rolando. 



A first glance at an adult human brain reveals what appears 

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

 when one 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, 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. 486 gives a representation of the brain of one of the 

 higher monkeys, the orang-outang, where there is an inter- 

 mediate 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 



i . The fissure of Sylvius ; this divides into two limbs, the pos- 

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



