MR. W. BEVAH LEWIS OR THE COMPARATIVE 
704 
1. Peripheral cortical zone. 
2. Small pyramidal layer. 
3. Ganglionic layer. 
4. Spindle cell layer. 
1 . Peripheral cortleal zone. —-As seen by the naked eye this is a uniformly light 
grey belt sharply defined from the darker layers beneath. Its structure, revealed by 
microscopic aid, does not differ from that of higher animals, being constituted of a fine 
neuroglia matrix derived from the delicate prolongations of connective cells supporting 
the meshwork formed by the repeated sub-divisions of the apical processes of nerve- 
cells in the underlying strata. We meet here, as in the Sheep, Pig, and other animals, 
with the Deiter cell. These cells are found immediately beneath the pia-mater, their 
processes extending downwards into this layer and a larger process invariably being 
connected with a nucleus along the sheath of a blood-vessel. As regards depth there is a 
very evident diminution of this layer upwards and outwards over the exposed aspect of 
the hemisphere as well as backwards towards the occipital pole. Thus in regions of 
this upper limbic arc anterior to the corpus callosum the depth is *511 mm., whilst 
posterior to this commissure it scarcely attains to *377 mm. A still more notable 
diminution in depth occurs outwards, viz.: in extra-limbic regions where this outer 
cortical zone measures but *279 mm. in depth. In the cortex of the Pig’s brain a 
similar fact was noted and is recorded in the table of depths of the cortical layers given 
in my former paper. * 
2. Small pyramidal layer.— In the upper limbic arc of the brain of Man and of 
some of the higher Mammalia I have described the typical formation as that of a five- 
laminated cortex. In the same region, however, of the Babbit’s brain the cortex is four- 
laminated, and this arises from the absence of the small oval and angular elements which 
in Man constitute the second layer. In certain localities, more especially extra-limbic in 
position, a shadowing forth of this stratum is represented by the closer aggregation of 
elements superficially which become also appressed in small clumps at intervals and 
may possibly represent a rudimentary form of this layer. So little marked is this in the 
upper limbic arc that it may virtually be considered devoid of this layer. The second 
layer, then, is in this region constituted by a stratum of small pyramidal nerve-cells as 
well as a few scattered angular cells. In the size of these elements, in their mode of 
distribution, in the depth of the stratum and its relationship to subjacent layers a very 
close resemblance is borne to the upper half of the third layer of higher animals. In 
the pyramidal cells of the human cortex a notable increase in their dimensions occurs 
with their depth so that at the lowest level of this series in Man I find cells whose 
proportions, though not usually so great as the cells of the still lower ganglionic layer, 
still render them giants beside their pigmy representatives higher up. This increase 
in size is gradually attained.! Now this feature is wholly absent in the small 
* Op. cit., p. 62. 
| “ The Cortical Lamination of the Motor Area of the Brain.” Proceedings Roy. Soe., Ho. 185, 1878. 
