246 THE ESSENTIALS OF HISTOLOGY. 



small granular-like cells amongst them (layer of larger pyramids). The 

 pyramidal cells are larger in the deeper parts of the layer. 



4. A layer of small irregular polymorphous cells. In the psycho- 

 motor region of the cortex (portions of the frontal and parietal lobes) 

 pyramidal cells of very large size extend amongst these polymorphous 

 cells, and are disposed in small clusters or " nests " (Be van Lewis, Betz) 

 (figs. 279, 280). 



5. A layer of small scattered cells, many of a fusiform shape. This 

 layer lies next to the white centre. It is not always distinct from the 

 polymorphous layer. In the island of Reil this stratum is considerably 

 developed, and is separated from the rest of the grey matter by a layer 

 of white substance. It is here known as the daustrum (see fig. 285, d.). 



From the white centre bundles of medullated nerve-fibres pass in 

 vertical streaks through the deeper layers of the grey matter, to lose 

 themselves amongst the pyramidal cells of the more superficial layers. 

 Some of these fibres are continuous with the axis-cylinder processes of 

 the pyramidal and polymorphous cells, and therefore take origin in the 

 cortex ; others are passing into the cortex to end amongst the cells of 

 the several layers in free arborisations. The axis-cylinder processes of 

 the pyramidal cells pass into the white centre. Here some of them are 

 continued either directly or by collaterals into the corpus callosum, 

 and go through this to the cortex of the opposite hemisphere (comwiv- 

 sural fibres) ; others join association fibres which run longitudinally or 

 transversely, eventually to pass again into the grey matter of other 

 parts of the same hemisphere ; whilst others again, especially those of 

 the largest pyramidal cells, extend downwards through the corona, 

 radiata and internal capsule, and become fibres of the pyramidal tract 

 (projection fibres). As they pass down through the white matter of the 

 hemisphere they give off collateral fibres to the corpus callosum and 

 to the basal ganglia (corpus striatum and optic thalamus) (see diagram, 

 fig. 253, p. 222). 



There is, as already stated, a great amount of variation met with in 

 the relative extent of development of the above layers. Some of these 

 variations are exemplified in the accompanying drawings of preparations 

 from the monkey's cerebral cortex by Bevan Lewis. From these it will 

 be seen that smaller sized cells prevail in the sensory regions of the 

 cortex (occipital, temporal) ; larger and fewer cells occur in the psycho- 

 motor parts. The structure of the hippocampal region presents so 

 many peculiarities as to necessitate a special description. 



In the hippocampus major and hippocampal gyrus (fig. 283) the 

 superficial layer of neurolgia and the white stratum, which overlies it 

 as a thin band in other parts of the cortex, are both very strongly 



