586 NERVOUS SYSTEM. 



of the sulci between them, is composed oF gray matter, which is here called 

 the cortical substance. The interior of each convolution is composed of white 

 matter ; and white fibres also blend with the gray matter at the sides and bottom 

 of the sulci. By this arrangement the convolutions are adapted to increase the 

 amount of gray matter without occupying much additional space, while they 

 also afford a greater extent of surface for the termination of the white fibres in 

 gray matter. On closer examination, however, the cortical substance is found 

 subdivided into four layers, two of which are composed of gray and two of 

 white matter. The most external is an outer white stratum, not equally thick 

 over all parts of the brain, being most marked on the convolutions in the longi- 

 tudinal fissure and on the under part of the brain, especially on the middle lobe, 

 near the descending horn of the lateral ventricle. Beneath this is a' thick 

 reddish-gray lamina, and then another thin white stratum ; lastly, a thin stratum 

 of gray matter, which lies in close contact with the white fibres of the hemi- 

 spheres; consequently white and gray laminas alternate with one another in the 

 convolutions. In certaia convolutions, however, the cortical substance consists 

 of no less than six layers, three- gray and three white, an additional white 

 stratum dividing the most superficial gray one into two ; this is especially 

 marked in those convolutions which are situated near the corpus callosum. 



There is no accurate resemblance between the convolutions in different brains. 

 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 sulci are generally an inch in depth ; they also vary in different brains, 

 and in different parts of the same brain; they are 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 mam- 

 malia, 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 arrangement, 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 supraorbital convolution, and the convolutions of the outer surface 

 of the hemisphere. 



The convolution of the corpus callosum (gyrus fornicatus) is always well marked. 

 It lies parallel with the free surface of the corpus callosum, commencing, on 

 the under surface of the brain, in front of the anterior perforated space; it 

 winds round the curved border of the corpus callosum, and passes along its 

 upper surface as far as its posterior extremity, where it is connected with the 

 convolutions of the posterior lobe; it then curves downwards and forwards, 

 embracing the cerebral peduncle, passes into the middle lobe, forming the hip- 

 pocampus major, and terminates ju.st behind the point from whence it arose. 



The supraorbital convolution on the under surface of the anterior lobe is well 

 marked. 



The convolution of the longitudinal fissure bounds the margin of the fissure on 

 the upper surface of the hemisphere. It commences on the under surface of 

 the brain, at the anterior perforated space, passes forwards along the inner 

 margin of the anterior lobe, being here divided by a deep sulcus, in which the 

 olfactory nerve is received; it then curves over the anterior and upper surface 



