THE TELENCEPHALON. 



1181 



dendritic processes, one passing outward and the other toward the subjacent third layer, on 

 entering which it divides and gives off the axone. At about the junction between the layer of 

 small and large pyramidal cells, the stripe of Gennari is produced by a close felt-work of 

 meduUated fibres, beneath which the pyramidal cells very gradually increase in size. In the 

 deepest part of the third and adjacent part of the fourth layer, pyramidal cells of unusually 

 large dimensions occur singly or in small groups. The layer of polymorphic cells is well 

 represented. 



The cortex of the hippocampus and of the gyrus dentatus is a prolongation of that of the 

 gyrus hippocampi, modified by the peculiar folding which here occurs. Reference to Fig. 992 

 will recall the relations of these gyri as seen on the mesial surface, namely, that at the bottom 

 of the deep groove (the hippocampal fissure) above the hippocampal convolution lies the corru- 

 gated free surface of the dentate gyrus and above this the rounded mesial border of the hippo- 

 campus. Viewed in cross-section (Fig. 1018), the cortex of the hippocampal convolution is seen 

 to bend laterally and pass into that of the hippocampus, which arches upward, mesially and 



Fig. 1019. 



Stratum 



granu- 



losuni 



Tangen- 

 tial zone 



circum.'' 



Stratum 

 radiatum 



Stratum 

 lucidum 





Free sur- 

 face of 

 gyrus 

 dentatus 



Nucleus 



(vJbfi Subicu- 

 ^^ " X lum 



Part of frontal section across left hippocampus and gyrus dentatus, showing arrangement of cell-layers. X 15. 



then, turning sharply laterally, blends with the dentate g>TUS, which recurves mesially to reach 

 the free surface of the hemisphere and fill the recess between the hippocampal g>'rus and the 

 under surface of the hippocampus. The cortex of the hippocampus, therefore, is folded upon 

 Itself somewhat like the curve of an interrogation mark. On approaching its upper convexity, 

 the cortex of the hippocampal convolution, here called the subiculmn, becomes modified by 

 the excessive but unequal thickening of the tangential fibre-layer of its stratum zonale and the 

 irregularity of its layer of small pyramidal cells, the large pyramidal cells at the same time 

 becoming the sole representatives of the third stratum. The laver of tangential fibres, some- 

 what thinned, passes onto the hippocampus which it follows throughout and comes, therefore, 

 into apposition with the corresponding tangential zone of the dentate gyrus. The two fibre- 

 layers are so blended that a differentiation between ihe two is impracticable. Beneath (i) the 

 layer of tangential fibres lies a second stratum of medullated fibres, (2) the lamina medullaris 

 ctrcumvoluta, which is probably an intracortical association tract limited to the hippocampus. 

 The zone succeeding the medullary lamina is penetrated by innumerable long dendritic pro- 

 cesses of the large pyramidal cells and in consequence presents a radial striation, the layer 



