1 02 
MR. W. BEVAN LEWIS ON THE COMPARATIVE 
Upon separating the olfactory lobe from the frontal end of the hemisphere and 
reflecting it backwards, we become aware of the fact that the frontal extremity is 
obliquely truncated, sweeping forwards and upwards to the frontal pole, and thus 
forming an oblique depression in which lies the opposed olfactory lobe. Into this part 
of the hemisphere runs the upper or fourth olfactory fasciculus. If we direct our 
attention to the base of the brain in the Babbit, on the other hand, we observe that 
the great limbic lobe does not, as in the Bat, conceal by its extension outwards the 
extra-limbic portion of the hemisphere, a large portion of which projects on either side 
of the well-marked limbic fissure. The olfactory lobe runs directly forwards beneath 
the frontal extremity of the hemisphere, its external or superficial fasciculus coursing 
backwards parallel to, and 2 mm. distant from, the limbic fissure (Plate 49, fig. 1, M), 
until at the gyrus hippocampi it curves inwards and is lost to view. On the median 
aspect of the Babbit’s brain the upper limbic arc is deep anteriorly, but narrowed 
behind, and curving down behind the corpus callosum, forms a deep fossa, here over¬ 
lapped by the occipital pole in which lies the quadrigeminal bodies. Below this 
it becomes continuous with the gyrus hippocampi or lower limbic arc (Plate 49, 
fig. 1, B). Along the upper arc two delicate furrows are seen, the representatives of 
the sub-frontal and sub-parietal segments of the limbic fissure (Plate 49, fig. 1 , A). 
In the Bat, however, there is no trace of any such furrows, the upper arc being here 
directly continuous with the sagittal portion of the parietal lobe. In both animals 
the Sylvian fissure is nearly absent, being indicated by a very slight depression betwixt 
the gyrus hippocampi and the olfactory region (Plate 49, figs. 1, 2, S). 
Section I. — The Cerebral Cortex. 
The complete cortical envelope of the cerebral hemispheres in the animals under con¬ 
sideration (and this with special application to the Babbit and the Bat) may be readily 
mapped out into six distinct regions characterised by a notable diversity in their 
laminar constitution (Plate 49, fig. 1). These regions, abruptly marked off from each 
other by then- topographical relationships as well as their intimate structure, are the 
upper limbic arc, the lower and anterior limbic arcs, the extra-limbic lobe, the olfactory 
bulb, and the cornu ammonis ; but whereas the two former of these regions are subject 
to further modifications of their textural laminations—modifications which are not 
simply those of degree, but of kind —it results that I must extend my classification 
of the varied cortical realms beyond the limits mapped out by former authorities. 
Thus MeynErt distinguishes five types of cortical lamination as follows :— 
1 . A common type. 
2 . Occipital type. 
3. Sylvian type. 
4 . Type of cornu ammonis. 
5. Type of olfactory bulb. 
