172 THE CEREBEAL HEMISPHERES, 



over, in their longitudinal course they give off collaterals here and there to inter- 

 mediate parts of the cortex. The commissural fibres, after passing through the 

 corpus callosum, have a longer or shorter course in the white matter of the opposite 

 hemisphere, and end by terminal arborisations in its cortex. In their course they also 

 give collaterals to other parts of the cortex. Some of the commissural fibres are 

 themselves collateral branches of the projection- fibres (fig. 20, p. 23). 



The projection-fibres pass for the most part into the internal capsule. Both 

 before reaching this and as they pass through it they give off collaterals, some of 

 which enter the basal ganglia and ramify amongst the cells, whilst others pass 

 towards the corpus callosum. The main fibres ultimately end by arborisation in 

 the grey matter of the lower nerve-centres : those of the pyramidal tract amongst 

 the cells from which the motor nerve-fibres originate. The projection fibres are 

 mainly derived from the middle-sized and larger pyramids, and from some of the 

 polymorphous cells of the fourth layer. 1 



Differences of structure in different parts. Considerable variety occurs in 

 different parts of the cerebral cortex in the size and regularity of shape of the nerve- 

 cells and in the relative thickness of the several layers. It is especially worthy of 

 notice that in the " psycho-motor " region, and particularly in the upper part of the 

 ascending frontal convolution, some of the deeper pyramidal cells are very large 

 (fig. 121, 4), and are arranged in more or less defined groups or nests (Betz, Bevan 

 Lewis). On the other hand, in the neighbourhood of the calcarine fissure, large 

 cells are very scanty, their places being for the most part taken by smaller ones 

 (fig. 118). Again, in many parts a six-laminated cortex is produced by the interca- 

 lation of a layer of small angular cells in the middle of the layer of large pyramids 

 (figs. 118, 119). 



These differences in the size and arrangement of the cells in different parts of 

 the cortex are well illustrated in the accompanying figures (118 to 123) by Bevan 

 Lewis of sections from various regions. 



The most remarkable differences of structure occur at the margin of the hemi- 

 sphere in the region of the hippocampus, and in the olfactory lobe. These will now 

 be specially considered. 



Structure of the hippocampus major or cornu ammonis. The hippo- 

 campus it will be remembered corresponds to the hippocampal fissure externally ; 

 this fissure separating the uncinate or hippocampal convolution below from the 

 fascia dentata above. The uncinate convolution (fig. 123) has the ordinary structure 

 of the cerebral gyri, being composed of a grey cortex and a thick white centre. 

 The cortex generally is, however, thin, but the superficial layer is relatively thick, 

 and contains many medullated fibres. The cells of the second layer are relatively 

 large as compared with other parts of the cortex. The cortex is prolonged around 

 the hippocampal fissure, forming the main part of the hippocampus (figs. 110, 124) ; 

 the white centre is also prolonged over the projection of the hippocampus into the 

 ventricle, but becomes very thin in this situation where it is known as the alveus. 

 It is covered by the epithelium and ependyma of the ventricle. 



Above the hippocampal fissure the grey matter of the hippocampus swells out 

 into the notched lamina known as the fascia dentata (dentate convolution, fig. 124, 

 F d}. The white matter of the alveus is in like manner prolonged over this, but not 

 quite as far as its free border ; it becomes thickened and is continuous with the 

 white band known as the fimbria (Fi), which thus represents a free edge of the 

 medullary centre of the hemisphere. 



The projection of the hippocampus (into the ventricle) is thus produced by the 



1 Most of the above details relating to the destination of the axis-cylinder processes are derived from 

 the observations of Ramon y Cajal, made upon specimens prepared by Golgi's method. 



