i ib2 HUMAN ANATOMY. 



being appropriately termed (3) the stratum radiatum. Following this comes (4) the layer 

 of pyramidal cells. These are uniformly of large size and closely packed within a clear 

 ground-work which confers a light appearance upon the winding lamella, which is therefore 

 sometimes known as the stratum lucidutn. Beneath the pyramidal cells lies a layer of fibres, 



(5) the stratum oriens, which pass to and from the hippocampus; among these fibres are 

 embedded spindle cells, as well as peculiar association cells (Cajal) possessing richly branched 

 axones which ramify among the pyramidal cells which they probably serve to link together. 

 The axones of the pyramidal cells are directed chiefly towards the centre of the gyrus where, 

 next the descending horn of the lateral ventricle, they form a conspicuous layer of fibres called 



(6) the alveus. It is this sheet, covered by (7) the ventricular ependyma, in connection with the 

 stratum oriens, which confers the white color to the hippocampus, as seen within the ventricle. 

 On reaching the recurved end of the hippocampus, the layer of pyramidal cells of the latter 

 is not continuous with that of the dentate gyrus, but ends irregularly and is enclosed by the 

 arched dentate cell-layer. 



The cortex of the gyrus dentatus is highly modified and less in accord with the typical 

 structure of the cortical substance than that of the hippocampus. The outer surface where 

 buried in the concavity of the hippocampal arch lies in contact with the similar surface of the hip- 

 pocampus, hence the peripheral layers of the two gyri are opposed. Within the gyrus dentatus 

 may be recognized ( i ) the stratum zonale, relatively narrow and meagre in fibres. The surface 

 of the gyrus is paralleled by a narrow layer of small and densely packed cells, (2) the stratum 

 granulosum. These almost, but not quite completely, surround the gyrus and, therefore, leave 

 an interval, the hilum, through which the fibres gain and leave the deeper parts of the convolu- 

 tion. Within the area so circumscribed, known as (3) the nucleus of the gyrus, are found 

 irregularly disposed elements, the representatives of the layer of large pyramidal cells. They 

 are for the most part small in size and atypical in form. Their axones, together with the 

 continuation of the stratum oriens, pass through the hilum, the dentate gyrus thereby forming 

 connections with other parts, either of the hippocampus or of the fimbria. 



THE WHITE CENTRE OF THE HEMISPHERE. 



The extensive medullary substance enclosed by the cerebral cortex appears, 

 above the level of the corpus callosum, as a grayish white tract (centrum semiovale*} of 



seemingly homogeneous structure, 



FIG. 1020. its uniform character being broken 



^^^^ ^mi^^ at most by minute blood-vessels. 



At lower levels, where the intercor- 

 tical area is encroached upon by 

 the large collections of gray sub- 

 stance composing the corpus stria- 

 turn and the thalamus, the white 

 matter is most conspicuous immedi- 

 ately subjacent to the cortex. When 

 examined with the microscope after 

 suitable preparation, the apparently 

 homogeneous subcortical tissue is 

 resolved into an intricate maze of 

 medullated nerve-fibres, supported 

 by neuroglia, which run in various 

 directions and are, therefore, cut in 

 different planes. When analyzed 

 as to their relations with the cortex, 

 the components of the medullary sub- 

 stance of the hemisphere fall into 

 three general groups : ( i ) the associ- 

 ation fibres, (2) the commissural 

 fibres, and (3) the projection fibres. 

 The Association Fibres. The association fibres link together different por- 

 tions of the same hemisphere, many uniting adjacent areas whilst others connect parts 

 widely separated. They arc grouped, therefore, mlong and short association bundles. 

 With the exception of a narrow zone in the immediate, virinity of the upper end of the 

 Riilandic fissure, the cerebral cortex at birth is unprovided with association fibres 

 which have acquired their medullary coat and, therefore, are capable of functioning. 



Frontal section of brain passing through hemispheres in 

 front of corpus callosum ; core of white matter is everywhere 

 enclosed by cortical gray matter. 



