STRUCTURE OF THE BRAIN IN RODENTS. 
743 
the far deeper staining which it takes. The olfactory arcuate fasciculi, therefore are 
wholly distributed to that portion of the median and superficial cortex of the occipital 
lobe which is characterised by the granule cell formation which here forms a deep 
second layer (Plate 49, fig. 1, K, D). This formation has been already fully described 
under the name of the modified upper limbic type. This region is moreover charac¬ 
terised by a further peculiarity, viz.: the two intra-cortical arcuate bands. The more 
superficial of these is a broad streak of medullated fibres which arising from the apical 
processes of the large cells in the cornu ammQnis, as its first layer (nuclear— Meynert) 
is continued along the lower half of the first cortical layer of the upper limbic arc, and 
bending over on to the upper surface of the hemisphere is almost immediately lost 
here. Beneath it lies a second shallower medullated band a little over half the depth 
of the former, and separated from it by the second layer of granule cells. This stripe 
also arises from the cornu ainmonis, and likewise running parallel to the cortex termi¬ 
nates even earlier than the more superficial stripe, as it ends abruptly at the upper 
marginal angle of the hemisphere. In such as we are now dealing with (fresh vertical 
sections), both these stripes may readily be seen by a hand lens running as delicate 
white streaks parallel to the surface of the cortex of the upper limbic arc, being lost 
beyond the upper marginal angle, whence no similar white streak can be detected 
until we arrive opposite the great mass of the projection system which here takes a 
lateral position. Here again we observe a delicate medullated stripe in the first 
cortical layer running towards the limbic sulcus, but lost prior to its arrival at that 
spot. This stripe is even shallower than the deeper arcuate stripe in the upper limbic 
arc. 
Region of expansion of central olfactory fasciculus. —It will be remembered that the 
termination posteriorly of the inferior limbic arc is characterised by the large size of 
the nerve-cells of its second cortical layer—the modified olfactory type. If horizontal 
sections of the occipital end of the frozen brain be carried through this region, the 
following facts may be readily confirmed. Most internally is the curved border of 
the cornu ammonis ; next to it comes the granule region, extended from the upper 
limbic arc ; still further outwards is the peculiar region alluded to ; and beyond its 
boundary (the limbic sulcus) is the curved surface of the extra-limbic portion of the 
hemisphere. 
To the naked eye a great dissimilarity is at once observed betwixt the appearance 
of the extra-limbic and the limbic portion. The former exhibits a medulla which has 
a slight translucent aspect and is of a pale tint, but markedly contrasted with the 
;perfectly opaque and brilliant white colour of the limbic medulla. The line of demar¬ 
cation, again, betwixt these two is sharp and abrupt, and extends from the depths of 
the corpus callosum directly outwards to the limbic sulcus. Microscopic examination 
clearly shows this difference in colour to be due to the vast preponderance of medullated 
nerve-fibres in the limbic region at this site, which radiate outwards to the cortex 
from the terminal portion of the callosal commissure. In the extra-limbic portion of the 
MDCCCLXXXII. 5 C 
