710 
MR. W. BEVAN LEWIS OR THE COMPARATIVE 
exposed aspect of the hemisphere, extends to the shallow parietal sulcus above described 
(Plate 49, fig. 1, K). If we follow this formation backwards it is found to occupy the 
whole remaining median aspect of the hemisphere as far back as the limbic sulcus.* 
From this point downwards its distribution is more and more restricted to the inner 
moiety of the limbic lobe until it finally dwindles into an insignificant tract to disappear 
at the posterior extremity of the gyrus hippocampi. The gradually narrowing tract as 
it comes downwards and inwards leaves an angular portion of cortex betwixt it and 
the limbic sulcus, whose apex corresponds to the extremity of the latter sulcus 
(Plate 49, fig. 1 , C). In this region is included the formation alluded to as the 
modified lamination of the inferior limbic arc. Vertical sections through the hemisphere 
behind the callosal commissure when stained by aniline black exhibit to the naked eye 
a remarkably defined dark belt corresponding to the granule belt just described 
(Plate 50, fig. 4, B). This belt can be traced outwards through the whole limbic arc 
and is seen to terminate abruptly at the parietal sulcus. On the other hand, fresh 
unstained vertical sections exhibit clearly to hand-magnifiers two delicate white stripes, 
one lying in the lower half of the first layer and the other lying within the granule belt 
of the second layer. These are found invariably in this region of the upper limbic arc 
(modified limbic type), but disappear as they bend over the marginal or sagittal angle 
to reach the exposed aspect of the hemisphere. They are constituted by two stripes of 
medullated fibres and will receive further attention when we come to describe the 
central projections of the olfactory organ. The depth of the individual layers of this 
modified upper limbic type immediately behind the corpus callosum was as follows :— 
1 . Peripheral cortical zone, ‘325 mm. to ‘372 mm. 
2 . Granule layer, ‘372 mm. 
3 . Ganglionic layer, '604 mm. 
4. Spindle layer, *604 mm. to '744 mm. 
The granule layer where deepest does not exceed *418 mm., nor does this forma¬ 
tion in any region attain the depth of the small pyramidal layer which it displaces 
and which latter usually measures '604 to '833 mm. deep. The substitution of this 
densely crowded, though shallow, layer for the deeper pyramidal series, together 
with the horizontal connexions rather than vertical, established by means of the 
various parallel arciform medullated belts, accounts for the notable shallowness of the 
whole cortex posteriorly as compared with anterior realms. Thus near the occipital 
pole the depth of the cortex is 1*737 mm., whilst in the neighbourhood of the frontal 
pole it is 2'577 mm. The granule cell is very small and averages 10/rX 8y, having 
a nucleus measuring 4/x to 5/x. A large number, however, scarcely reach the dimen¬ 
sions of 9/a X 9/a. In form they are conical, the nucleus constituting their chief bulk, 
whilst the very limited and delicate protoplasm extends irregularly into several pro¬ 
cesses, the most notable being usually, although by no means universally, apical. 
* Represented by dotted area on medium aspect of brain. Plate 49, fig. 1. 
