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MR, W. BE VAN LEWIS OR THE COMPARATIVE 
that of the opposite side, as a purely commissural system. This of course is in 
accordance with the fact indicated by Gratiolet, Meynert, and others, that in the 
Rodentia and Carnivora, where the olfactory apparatus is largely developed, the great 
bulk of fibres proceeding from the anterior commissure communicate with the olfactory 
lobe; whilst in Man and Apes, where its development is small, the larger portion of 
the commissure is distributed backwards to the occipital and to the temporo-sphenoidal 
lobes. The central olfactory fasciculus behind the anterior commissure exhibits a 
notable difference in its distribution in the Rabbit and the Rat, a divergence which has 
an important bearing upon the functional relationships of the olfactory lobes. In the 
Rat, as has just been stated, the fasciculus passes outwards through the corpus striatum, 
and immediately upon reaching its outer border divides into a brush-like head of nerve 
bundles, some of which probably unite' with the projection fasciculi passing through 
them up to the vault, but by far the most extensive portion turns back, and runs 
towards the occipital pole of the hemisphere (Plate 49, fig. 9). In the Rabbit, on the 
other hand, when the posterior olfactory fasciculus reaches the outer surface of the 
corpus striatum it does not, as in the Rat, break up into numerous diverging bundles, 
but bends upwards and ascends as a more or less compact fasciculus in contact with 
the outer surface of the ganglion (Plate 49, fig. 11, A). Upon reaching the level of 
the upper pole of the striate ganglion it meets with the accumulating mass of 
the projection and callosal systems (C, I)), and at this spot it suddenly breaks up 
into an extensive brush-like head of fibres (B), which pierce through the mass of the 
projection system, and are thence directed towards the cortex of the inner margin 
of the hemisphere, i.e., the margin bounding the great longitudinal fissure (E). 
In vertical sections which have been carried successfully through the plane of the 
posterior commissural fasciculus, we may readily trace it—first in its horizontal course 
through the striate ganglion, next curving round and up the outer aspect of that 
ganglion, and lastly on its arrival at the upper pole, grasping, as it were, by a mass of 
tentacular-like extensions, the base of the projection system, decussating freely with 
the callosal fibres, meeting them from an opposite direction, and with the projection 
fibres betwixt striate body and cortex, which in part traverse the same course.'' AUliat 
is the destination of this important system of fibres ? It appears indeed to form that 
extensive arcuate system which lies beneath the cortex of the summit of the hemisphere 
at these planes. This arcuate system may be readily seen lying betwixt the cortex of 
the vault and the great mass of the projection and callosal systems above the corpus 
striatum (Plate 49, fig. 11, F). They therefore are directed nearly at right angles to 
the radiating callosal fibres for the cortex of the vertex, and running parallel to the 
surface of the cortex, terminate in the region of the large ganglionic cells which have 
been referred to as seen in the cortex along the inner margin of the vault. In short, 
then, the olfactory bulbs of the Rabbit are thus brought into direct and decussating 
connexion through an extensive course with the specialised cortex of the inner 
* The whole of this course is represented in Plate 49, fig- 11. 
