742 
MR. W. BEY AN LEWIS ON THE COMPARATIVE 
the lowest throughout their course, and are placed next to the ganglia in their course 
to the cortex, which for them is the most external part of the hemisphere, i.e., the 
undermost portion of the corpus callosum terminates in the cortex just above the 
limbic sulcus externally. On the other hand, the uppermost series of fibres in this 
commissure connect the more internal aspects of the brain. A superficial section 
lengthwise through the corpus callosum would therefore deprive the median and inner 
regions of the brain of their commissural connexions, whilst the deeper the incision 
was carried the more external the regions of the hemispheres which would suffer 
likewise. 
Expansion of olfactory arcuate system into occipital cortex .—An important distri¬ 
bution of fibres from the olfactory cortex has been already traced into the occipital 
lobe lying like a longitudinal fasciculus (of inverted V-shaped form in vertical section) 
upon the prominent ridge formed by the apposition of the callosal and projection 
fasciculi (Plate 49, fig. 11, F). The ultimate destination of these fibres may best be 
studied in vertical sections through the hemisphere behind the posterior commissure. 
In such sections we expose the central medulla folded around the cornu ammonis, 
which in its turn lies concentric to the great central ganglia (Plate 49, fig. 10). This 
mass of central medulla reveals two important regions, (1) the region of section of the 
great mass of projection fasciculi which here is lateral in position (F); (2) the region 
of the callosal and arcuate olfactory fasciculi which is median in position (H). The 
latter with which we are now concerned is wedge-shaped, in such section the base of 
the wedge directed towards the median cortex, the two converging sides directed 
outwards. The central core, so to speak, of this wedge is constituted by the callosal 
fasciculi subdivided into an upper segment destined for the marginal angle and tip of 
occipital lobe, and a lower segment which is continuous outwards towards the lateral 
projection system radiating to the cortex throughout this course and the divided fibres 
of the fornix. Superficial to it and running in the opposite direction is a small band 
of projection fasciculi, which has also for its ultimate destination the cortex of the 
outer and upper surface of the hemisphere as far as the marginal angle. Upon the 
upper basal angle of the wedge rides the fasciculus of the olfactory arcuate and arcuate 
system of the upper limbic arc, which here have accumulated into a vast collection of 
fibres opposed to the whole base and upper side of the wedge. These fibres, however, 
are no longer found in transverse section, but are seen spreading extensively on all 
sides to the cortex (1) of the median aspect (upper limbic arc), (2) the marginal angle 
of the hemisphere, and (3) the cortex of the vault as far outwards as the superior 
parietal sulcus. Outside the latter sulcus the coronal distribution to the cortex con¬ 
sists of scanty projection and callosal fasciculi; within the sulcus, on the other hand, 
the cortex is supplied by dense callosal, a few projection fasciculi, and the notably 
large supply of olfactory and limbic arcuate fibres. This rich medullated region 
reveals its wealth of medullated fibres compared with the rest of the hemisphere when 
simply examined by the naked eye, and in osmic acid preparations shows the same in 
