516 



NERVOUS SYSTEM. 



nor are they symmetrical on the two sides of the same brain. Occasionally the 

 free borders or the sides of a deep convolution present a fissured or notched 

 appearance. 



The suki are generally an inch in depth, but they vary in different brains, and 

 in different parts of the same brain, being usually deepest on the outer convex 

 surface of the hemispheres ; the deepest is situated on the inner surface of the 

 hemisphere, on a level with the corpus callosum, and corresponds to the projection 

 in the posterior horn of the lateral ventricle, the hippocampus minor. 



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 

 their increasing complexity of arrangement as we ascend from the lowest mam- 

 malia 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 



Fig. 264. Upper Surface of the Brain, the Pia Mater having been removed. 

 Great longitudinal Fissure 



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. In old age, when the mental faculties have diminished in activity, 

 the convolutions become much less prominently marked. 



Those convolutions which are the largest and most constantly present are the 

 convolution of the corpus callosum, the convolution of the longitudinal fissure, 

 the supra-orbital convolution, and the convolutions of the outer surface of the 

 hemisphere. 



