7] 6 
MR. W. BEVAN LEWIS ON THE COMPARATIVE 
Extra-limbic mass. —Having now completed the examination of the great limbic 
lobe, I have next to consider the greater portion of the hemisphere which lies external 
to the constricting limbic arc, or that portion which in higher animals enters into the 
formation of a frontal, parietal, and tempo-spheroidal lobe. As in the detailed account 
of the various layers of the cortex in the upper four-laminated limbic arc I have 
anticipated many points bearing upon the structure of this extra-limbic region, it 
appears only necessary here to allude to the general points of divergence or similarity 
borne to other regions, and not to consider each individual layer minutely. The 
cortex, then, of this region* is five-laminated, consisting of similar layers of nerve- 
cells to those found in the upper limbic arc along with the interposition of a layer of 
small angular cells. The relative position of layers is as follows 
1. Peripheral cortical zone, 
2. Small pyramidal layer. 
3. Belt of granule or angular cells. 
4. Ganglionic series, 
5. Spindle cells. 
As regards diversity of cell formations, therefore, this is by far the richest region 
examined, and but a cursory glance at sections from near the occipital and frontal 
poles, as well as midway betwixt these points, will satisfy us that though this richly- 
laminated cortex has a most extensive distribution the varied layers differ much in 
their relative development in these different regions. Thus the small pyramidal layer 
is uniformly rich and deep, whilst the ganglionic series is especially developed in the 
anterior and median regions, and the granule and angular cells predominate in the 
posterior realms. Directing attention to the variations which occur in mid-cortical 
regions from within outwards, i.e., from the great longitudinal fissure outwards and 
downwards to the lower boundary of this region, the limbic fissure, I find first, that 
the outer cortical zone, or first layer, diminishes in depth, attaining its greatest 
shallowness near the limbic fissure; next, the small pyramidal layer increases in depth 
and richness of cells in the same direction ; thirdly, the ganglionic series tend to 
become distributed, after the laminar or solitary type, as we approach towards this 
region ;t whilst in the upper realms, near the median fissure, they retain the dense 
confluent aspect and depth of stratum which is likewise seen in the upper limbic arc. 
It will likewise be observed that the intercalated layer of angular elements commencing 
in the posterior half of the upper limbic arc extends over the far greater area of the 
extra-limbic mass, and progressively increases in richness of cells and distinctness of 
lamination outwards and especially backwards towards the occipital pole. As already 
stated, this type of lamination terminates abruptly at the limbic sulcus, the small 
pyramidal and ganglionic series alone being continued beyond into the lower limbic arc. 
* The five-laminated cortex of tliis region is the unshaded area at summit and sides, Plate 49, fig. 1. 
| The solitary arrangement is seen well in section from sulcus of limbic lobe of Pig, op. cit., plate 7. 
