732 
MR. W. BE YAW LEWIS OW THE COMPARATIVE 
deeply within the substance of the septum lucidum (g), whilst the cortex of the 
septum is seen to possess a double arciform belt., one superficial to its second layer (h), 
and the other beneath these cells (1c).' These arciform stripes communicate by 
vertical intermediate fibres with each other in their course upwards to the corpus 
callosum. 
From this olfactory area an important medullated fasciculus descends to enter the 
motor columns of the cord, hence affording a direct channel for the impulses propagated 
on excitation of the olfactory bulb. The double connexion with motor columns of 
the medulla and the cerebral cortex is the usual condition in Mammalia, although, 
as Broca has indicated, the connexion with the cortex alone is the only constant 
feature. 
3. Arciform fasciculi. —If the olfactory lobe be reflected downwards, away from the 
frontal pole, an upper medullated connexion betwixt the former and the hemisphere, 
may be traced lying invariably over the central canal of this lobe. But by far the 
more important arciform fasciculi form two series, which taking origin in the cortex 
of the olfactory lobe, are destined for the gyrus fornicatus and posterior regions of 
the brain. In vertical sections through the frontal end of the hemisphere, near the 
commencement of the caudate nucleus, the central olfactory fasciculus is seen in trans¬ 
verse section, imbedded in the head of this nucleus, which rises dome-like above it. 
Descending from the region of the central medulla of the hemisphere, are two medul¬ 
lary finger-like processes which appear to grasp the caudate nucleus betwixt them. 
The outer is constituted by fasciculi arising from the external borders of the caudate 
nucleus and disappears in the central medulla. The inner lies parallel with the 
central mass of medulla, but is wholly distinct from it, and constitutes the arciform 
fasciculi, which originate in the cortex of the olfactory lobe at the base and median aspect, 
and curving up the inner side of the hemisphere is closely connected with the cortex 
of the upper limbic arc. At the median aspect, where the upper limbic arc unites with 
the olfactory cortex, numerous medullated fasciculi arising from the latter follow the 
bend of the hemisphere, and thence arching strongly inwards come into relationship 
with the under surface of the corpus callosum, and turning backwards, course along its 
inferior aspect, whilst other fasciculi pierce its structure and join the arciform system 
of the gyrus fornicatus, which rests upon the upper surface of the corpus callosum. 
(Plate 49, fig. 12, 7 ). At its origin this important fasciculus is seen in vertical sections, 
ascending in a sigmoid curve, with the convexity first inwards and then outwards 
towards the under surface of the callosal commissure. A little further back the 
septum lucidum is found to the inner side of the olfactory area. In this animal it 
appears as a largely-developed structure, heart-shaped in vertical section, and formed 
of two lateral folioles uniting centrally, and not enclosing a ventricle betwixt them as 
in Man. Through the substance of the septum a large number of medullated fibres 
ascend to the inferior aspect of the corpus callosum (Plate 49, fig. 8, G). The 
longitudinal band formed by the arciform fibres of the olfactory and upper limbic 
