STRUCTURE OF THE BRAIN" IN RODENTS. 
727 
and surrounded by tire central medullary fasciculus, forming a direct communication 
between the olfactory lobe and the lateral ventricle. Still converging, these two 
fasciculi decussate in the median line immediately in front of the pillars of the fornix, 
constituting by this convergence the anterior commissure of the cerebrum, being in this 
latter course imbedded in the septum lucidum. The resultant fibres of this decussation 
pass out from the commissure as a compact fasciculus, and are not projected directly 
backwards, but arch suddenly outwards, passing through the substance of the corpus 
striatum. In this course the fasciculi of either side diverge so rapidly that they lie 
very nearly in one and the same line, a single vertical section through the hemispheres 
presenting us with the commissural band and both posterior fasciculi as far as the outer 
border of the striate body. To follow the further course of these posterior projections 
from the olfactory bulb, horizontal sections of the cerebrum of the Rat should be 
examined, and it will then be found that each fasciculus passes through the striate 
body, and upon reaching its most external border each olfactory band sub-divides into 
several secondary fasciculi similar in appearance to those which result from the union 
of the fibres of the first link of the projection system at their connexion with the 
receiving surface of the corpus striatum (Plate 49, fig. 9, A). 
These sub-divisions of the posterior olfactory fasciculus curve directly back into the 
occipital pole (Plate 49, fig. 9, B). If vertical sections of the hemisphere at this site 
be examined, the connexions of the cortex with the corpus striatum by the projection 
system of fibres will be found to be chiefly affected by the upper surface of the 
ganglion, with which a large bulk of coarse fasciculi are connected (Plate 49, fig. 10, A); 
yet the whole outer aspect of the ganglion in like manner receives numerous fasciculi, 
which form a complete medullated investment over its outer surface. This medullated 
investment being augmented by fewer fibres, and yet fewer at lower levels, eventually 
thins off into an insignificant tract (Plate 49, fig. 10, B). Now it is by no means 
improbable that a certain number of minute fasciculi in the Rat pass from the sub¬ 
divisions of the posterior olfactory fasciculus upwards along this medullated tract. I 
have repeatedly imagined that I could trace such a connexion, and if this turns out to 
be correct then we have here a distribution similar to although far more restricted than 
what I have assured myself is the case with the Rabbit’s brain. The position of this 
posterior olfactory fasciculus from the anterior commissure in its course through the 
corpus striatum is parallel to the base of the lenticular wedge, lying in fact betwixt its 
two main divisions, as may be clearly seen in horizontal sections taken at the proper 
plane, in which will also be very evident the fact, that whilst these fibres run directly 
backwards, the majority of the projection fibres of the corpus striatum pass directly 
outwards or forwards to the cortex (Plate 49, fig. 9, A—C), The olfactory fasciculus, 
prior to the formation of the anterior commissure, measures in its shortest diameter 
‘78 mm. to ’87 mm. ; the fasciculus emerging from the commissure, as it leaves the 
septum lucidum to enter the corpus striatum, measures but ‘52 mm. This reduction 
in bulk is due to the absence of a series of fibres which run from one olfactory bulb to 
5 A 
M DCCCI jXXXII. 
