744 
MR, W. BEY AN LEWIS ON THE COMPARATIVE 
hemisphere we likewise observe medullated fibres which radiate outwards to the cortex 
along its whole extent, and proceeding solely from the corpus callosum do not form 
such dense aggregations, but light radiating fibres some distance apart. In the limbic 
portion, however, the same fibres are present, plus a strong reinforcement from the 
posterior extensions of the anterior commissure, i. e ., the central olfactory fasciculus. 
Here then appears demonstrated an important fact, that the cortex of the modified 
olfactory type (Plate 49, fig. 1, T to W), receives medullated radiations so extremely 
rich in fibres that it is mapped out to the naked eye from the neighbouring extra 
limbic medulla by its notable white brilliant aspect, and that these fibres include the 
terminal expansion of the central olfactory fasciculus. 
Here again we meet with two intro,-cortical arciform stripes of medulla disposed as 
in the modified upper limbic arc, viz.:— 
(a) A superficial streak at the lowest confines of the first layer. 
(b) A deeper streak beneath the large swollen cells of second layer.* 
The superficial stripe is, however, here the shallower of the two, and the deeper 
stripe attains a width equal to the first band in the upper limbic arc. In horizontal 
sections the medullated fibres of these two streaks will be divided obliquely. In 
sections such as I have just been considering a rich series of arcuate fibres will also 
be found extending backwards through the extra-limbic lobe from the anterior realms 
of the brain, which thin out gradually towards the abrupt line of denser medulla, 
commencing at the limbic sulcus, and wholly disappear here. This is the arcuate 
system of the extra-limbic medulla. 
The above facts enable me to conclude that at the occipital pole of the hemisphere 
there are two important regions definitely mapped out by coarse external configuration 
of the brain and also by minute structural divergences. 
These regions are histologically distinguished by the possession of a cortex, which is 
peculiar to each, and which we have named respectively the modified upper and lower 
limbic types. Each region is associated with that of the opposite hemisphere by the 
callosal commissure. 
Each region is brought into relationship with the olfactory lobe and bulb; the 
upper limbic by means of the arcuate olfactory, the lower limbic through the medium 
of the central olfactory fasciculi after the decussation in the posterior commissure. 
Each region is moreover supplied with the peculiar double stripe of medullated 
fibres, forming arcuate systems within the cortex itself. 
Hence we have in these regions a reflection backwards of the olfactory sensory 
surface brought strongly into association with the cornu ammonis by means of the 
arcuate intra-cortical stripes. 
* The site of these intra-cortical hands of medulla is approximately given in Plate 50, fig. 6, a and b. 
