724 
MR. W. BEVAjST LEWIS ON THE COMPARATIVE 
Lnes betwixt regions differing in the structure of their cortex : these were the crucial 
and infra-parietal sulci, together with the fissure of Rolando. My later investigations 
enable me still further to extend this list by others, viz.: the limbic fissure, olfactory 
sulcus, and primary parietal sulcus. Thus in the Sheep and Pig we find that the 
crucial and sub-parietal sulci separates the five-laminated cortex, with its clustered 
ganglionic series, from the six-laminated cortex, with its solitary ganglion cells, the 
former lying in front and the latter behind these fissures. In like manner the inner 
extremity of the crucial sulcus formed the boundary betwixt the five-laminated cortex 
of the upper limbic arc anteriorly, and the peculiar granule formation of this arc behind 
the crucial sulcus, which extends as far as the retro-limbic annectant. In the Rat and 
Rabbit it is again observed that the limbic fissure, in separating the lower limbic arc 
from the parietal or extra-limbic mass, sharply defines two entirely distinct types of 
cortical lamination. Above the fissure lies the Jive-laminated cortex of the extra-limbic 
mass, with its intercalated series of granule cells; beneath it lies the tliree-laminated 
cortex of the outer olfactory region and gyrus hippocampi, whilst still further back 
there is found internal to it the remarkable modified olfactory cortex which has been 
described near the occipital pole. Within the great limbic lobe we find a sulcus, 
dividing two important and morphologically distinct regions. These regions are the 
outer and inner olfactory areas whose boundary line is the olfactory sulcus, wherein 
lies imbedded the superficial olfactory fasciculus. It was moreover shown that the 
superior parietal sulcus forms a sharp line of demarcation betwixt the five-laminated 
type of the parietal mass and the peculiar dense granule formation of the posterior or 
modified upper limbic cortex. In the higher animals, as was indicated in the former 
paper, the fissure of Rolando, here lying at the frontal pole, separates a five from a 
six-laminated cortex. We surely have here a most important fact, and one which, if 
followed up in further researches, will enable us to map out the convolutionary surface 
of the brain after a less arbitrary plan than the one usually adopted. The more fully 
I investigate the minute structure of the cortex and its deep connexions, the more 
forcibly am I impressed by the belief that the various fissures and sulci are not mere 
accidental productions, but have a deep significance of their own, dividing off the 
cortical superficies into morphologically if not physiologically distinct organs. Hitherto 
the fissures and sulci which I have found to be the boundary lines of distinct cortical 
realms are the following :— 
1. The limbic fissure. 
2. The infra-parietal sulcus. 
3. The crucial sulcus. 
4. The superior parietal sulcus. 
5. The inter-parietal sulcus. 
6. The olfactory sulcus. 
7. The fissure of Rolando. 
