STRUCTURE OF THE BRAIN IN RODENTS. 
725 
In the lowest of the animals examined, the Rat and Rabbit, we find the anterior 
lower limbic arc already well developed, distinct from surrounding regions, and forming 
the far greater bulk of the great limbic lobe. We find the limbic fissure deep and 
complete as far back as the region where in higher animals the retro-limbic annectant 
unites the limbic to the occipital lobe. Within this fissure lie the three distinct cortical 
regions of the outer, inner, and modified olfactory types. On the other hand the 
upper limbic arc in these animals is far from being so definitely separated from the 
neighbouring regions of the extra-limbic mass. The anterior or four-laminated seg¬ 
ment of this arc is, in fact, directly continuous with the exposed cortex of the vault, no 
fissure, sulcus, or depression however small intervening. In connexion with this fact 
it is, however, important to bear in mind the identical nature of the laminated type of 
this portion of the limbic arc and the exposed margin of the hemisphere which bounds 
the longitudinal fissure (sagittal). The modified limbic type of the posterior segment 
of this arc, however, is sharply defined from the five-laminated cortex of the extra- 
limbic mass by the primary parietal sulcus. In the Pig and Sheep, on the other hand, 
whilst the bulk and differentiation of the lower limbic arc is maintained, there is a 
decided advance in this respect evident in the upper limbic arc also. The anterior 
portion of this arc, from its size, actually overlaps so as to appear upon the upper 
surface of the hemisphere, whilst a most characteristic and deep sulcus, the crucial, 
separates it sharply from the complex convoluted surface of the parietal lobe.* The 
oblique course of this sulcus carries it inwards to the median aspect of the hemisphere, 
midway betwixt frontal and occipital pole, where it meets the subparietal sulcus, 
regarded by Broca as the continuation upwards of the limbic fissure. This latter 
sulcus, sweeping backwards, does not join the limbiG fissure posteriorly, being separated 
by the retro-limbic annectant, but as the latter annectant corresponds to the region 
which in the Rat and Rabbit is characterised by the modified olfactory type of cortex, 
this sub-parietal sulcus forms a line of demarcation betwixt the latter type of cortex 
and the granule formation of the upper arc. The mapping out of distinct morpho¬ 
logical areas exhibited in the lower limbic arc of the smooth brain of the Rat and 
Rabbit, is a feature which in the convoluted brain of the Pig and Sheep becomes 
extended to the upper limbic arc also through the medium of the crucial and sub- 
parietal sulci. Moreover in these convoluted brains the deep inter-parietal sulcus also 
divides two highly characteristic formations. Still higher, as in the Carnivora, the 
fissure of Rolando constitutes a similar limitary zone, the cortex of the ascendino* 
parietal (post-Rolanclique of Broca) being six-laminated; that of the ascendino* 
frontal five-laminated. 
* Op. cit. 
