STRUCTURE OF THE BRAIN IN RODENTS. 
731 
limbic arc, and is sharply separated from the former area by the superficial olfactory 
fasciculus, whilst on its outer side it is of course bounded by the limbic sulcus. Follow¬ 
ing exactly the outer border of the pyriform area, the superficial olfactory band is 
naturally at first curved outwards, and then inwards towards the Sylvian depression. In 
sections taken across a slightly higher level the same regions are distinctly seen and 
their structure rendered clear. The innermost area is now seen to consist of numerous 
oblong and elliptic masses of grey matter enclosing nerve corpuscles ; these grey nuclei 
vary much in size, but usually measure ‘348 mm. to '870 mm. in length by '204 mm. 
to 300 mm. in width. They are closely embraced by bundles of medullated fibres, 
which form dense fasciculi betwixt them and are all directed from before backwards. 
They originate in the cortex of the olfactory lobe and pass through this ganglionic region. 
Mingled with these is a separate system of fibres which arise from the granule layers 
of the olfactory bulb, and coursing chiefly along the inner basal aspect of the olfactory 
lobe also enter this olfactory area, but do not form fasciculi. Their peculiarity consists 
in their running as separate fibres very irregularly, as bending, decussating and crossing 
each other in various directions so as to give the impression of their forming an open 
meshwork. These nerve fibres measure "002 mm. to ’004 mm. in diameter. They 
penetrate the oval nuclei, and are found throughout their substance. As they do not 
extend beyond this region they terminate almost certainly in the nerve-cells of its grey 
masses. The outer surface of this olfactory region is covered, as was indicated by 
Meynert, by a thin layer of cortex, whose structure has already been fully described 
(p. 714). In passing through the olfactory area this deep mass of medulla is disposed 
in numerous longitudinal divisions or clumps well shown in vertical sections, from 
which smaller fasciculi are seen to descend to the cortex of the base, where they 
become apparently continuous with the arciform medulla superficial to and beneath the 
second cortical layer. Continuous with this deep olfactory medulla is a minor segment 
given off from the superficial olfactory fasciculus along its inner margin, and which can 
be best demonstrated by longitudinal sections through the hemisphere. The following 
scheme represents in a diagram the numerous systems of medullated fibres seen in 
vertical sections through this olfactory area (Plate 49, fig. 8). In the first place, we 
trace the cortex from the limbic fissure ( a) downwards beneath the superficial olfactory 
fasciculus ( b ) prolonged over the basal aspect of the caudate nucleus. Its second layer 
is seen disposed in the peculiar duplicatures described, and beneath which he the 
clumps of deep olfactory medulla seen in cross section (to) forming massive fasciculi 
contrasted strangely with the more distant fasciculi of the corpus striatum proper. 
Betwixt the deep medulla and the second layer is the deep arciform belt ( d ) with 
its great spindle cells apparently continuous with the claustral formation exter¬ 
nally (c) and with the arciform medulla passing up the septum lucidum internally (e). 
Descending from the deep medulla into this arciform belt are numerous vertical 
fasciculi (/), which may also probably form connexions with the second layer of the 
cortex. The deep medulla gives off laterally a dense mass of fibres which ascend 
