THE BKAIX AXD ITS 



773 



The terms " fissure' and "sulcus" are often used interchangeably. 



The Gyri or Convolutions. There is no accurate resemblance between the con- 

 volutions in different brains, nor are they exactly symmetrical on the two sides of 

 the same brain, but their general arrangement or plan is fairly constant. Certain 

 infoldings of the cerebrum take place at an early period of development and form 

 important landmarks, which are constant and can without difficulty be recognized, 

 but the secondary depressions and convolutions vary considerably. 



The number and extent of the convolutions, as well as their depth, appear to 

 bear a close relation to the intellectual power of the individual, as is shown in 



Lower 

 frontal 



fissure of 

 ~ Rolando. 



'Porieto-occtptfcrf fiuure. 

 FIG. 4&4. Upper surface of the brain, the arachnoid having been removed. 



their increasing complexity of arrangement as we ascend from the lowest mammalia 

 up to man. Thus they are absent in some of the lower orders of mammalia, and 

 they increase in number and extent through the higher orders. In man they 

 present the most complex arrangement. Again, in the child at birth, before the 

 intellectual faculties are exercised, the convolutions have a very simple arrange- 

 ment, presenting few undulations, and the sulci between them are less deep than 

 in the adult. 



The convolutions on the outer convex surface of the hemisphere are the largest 

 and most complicated ; their general direction is more or less oblique ; they fre- 

 quently branch like the letter Y in their course upward and backward toward the 

 longitudinal fissure: these convolutions attain their greatest development in man, 

 and are especially characteristic of the human brain. 



