644 THE NERVOUS SYSTEM. 



are followed to the various gyri of the cerebral hemisphere. This arrangement 

 is termed the corona radiata. The callosal system of fibres, as they proceed into 

 the hemisphere, also radiate, and they intersect the fibres of the corona radiata 

 (Fig. 576, p. 649). 



External Capsule. The thin lamina of white matter between the lateral aspect 

 of the putamen and the claustrum is called the external capsule. This joins with 

 the internal capsule in front of and behind the putamen, and in this manner the 

 nucleus lentiformis is encapsuled by white matter. 



INTIMATE STRUCTURE OF THE CEREBRAL HEMISPHERE. 



The cerebral hemisphere is composed of an external coating of gray matter, 

 termed the cortex, spread over an internal mass of white matter, which is called the 

 medullary centre. The cortex is of peculiar interest, seeing that there is good reason 

 for believing that in it the higher functions of the brain, or those which may be 

 classed under the general designation of the intellectual functions, take place. It 

 is within the same layer of gray matter that the influence of those external impres- 

 sions, which gain access to the cerebro-spinal axis through the senses, finally take 

 shape as consciousness ; and in it are placed also the centres which carry on the 

 psycho-motor functions. The white medullary centre is composed of nerve-fibres 

 which constitute the paths along which the influence of impressions is carried to 

 and from the cortex, and from one part of the cortex to another. 



THE CEREBRAL CORTEX. 



The gray cortex is spread over the entire surface of the cerebral hemisphere, but 

 it does not form a layer of equal thickness in all localities. At the summit of a 

 gyrus it is apt to be thicker than at the bottom of a furrow. The maximum 

 thickness of cortex (about 4 mm.) is attained in the superior parts of the motor 

 area, whilst the minimum (about 1/25* mm.) may be observed in the region 

 of the occipital pole. The amount of gray cortex differs considerably in different 

 individuals, and appreciably diminishes in old age. It is also stated, but upon 

 imperfect evidence, that it is relatively more abundant in the male than in the 

 female. 



In structure, likewise, marked differences may be noted in the gray cortex of 

 different regions, and much has been recently done in the direction of pointing out 

 the connexion of these structural peculiarities with the functional characteristics of 

 particular areas and applying them to the determination of the significance of the 

 furrows that subdivide the cerebral cortex into a series of ridges or gyri. This 

 structural difference is quite apparent to the najjed eye when sections are made 

 through the cortex in a fresh brain, and sharp transitions in structure occur at the 

 place where one area joins another. It is only to those general structural feature 

 which more or less characterise the entire cortical layer that we shall be able to refer. 



When sections are made through the fresh brain, and the cut surface is closeb 

 inspected, it will usually be apparent that the cortex is distinctly stratified, 

 the outside there is a thin, whitish layer, and beneath this the gray matter presenl 

 two strata of very nearly equal thickness, viz., a middle, gray-coloured stratum ai 

 an inner, yellowish-red stratum. Between the two latter layers a narrow whit 

 band is, in many places, visible. This is termed the outer band of Baillarger. Whe 

 the layers indicated above are present, four strata, superimposed one upon the othe 

 are recognised ; but in certain regions a second white streak traverses the deep 

 inner gray layer and divides it into further stratifications. This is termed the i] 

 white band of Baillarger, and, when it is present, the gray cortex becomes divide 

 obscurely into six alternating white and gray layers. 



The outer band of Baillarger is strongly marked in the region behind 

 calcarine sulcus and gives a characteristic appearance to this portion of the corte: 

 In this locality it receives the name of the stria of Gennari (Fig. 567, p. 638). 

 White Medullary Centre of the Cerebral Hemisphere. The white matter of tl 



