STRUCTURE OE THE BRAIN IN RODENTS. 
733 
cortex is distributed eventually to a peculiar region already described, near the occi¬ 
pital pole of the hemisphere, which is covered by cortex of the modified upper limbic 
type. This termination will be described when I refer to the central expansions of 
the olfactory medulla. 
4. Superficial olfactory fasciculus (“External root”).—This fasciculus appears, 
from a surface view, as a flattened white lamina covering the basal aspect of the 
olfactory lobe. Widest in front, it encircles the greater surface of this lobe, grasping 
its lower, outer, and portion of its upper aspect. Thence directed backwards, it sweeps 
round the olfactory area, forming, as already stated, its outer boundary, and becoming 
rapidly attenuated, runs as a delicate white streak inwards, in a direction which, 
if continued, would pass nearly at right angles across the origin of the optic tracts. 
In this, its latter course, it passes over that slight furrow which here indicates the 
Sylvian fissure, and is lost in the substance of the gyrus hippocampi. Vertical 
sections show that this fasciculus is not a mere flattened lamina, but a thickened 
oval or oblong band, which is sunk to a little depth in the peripheral zone of the 
cortex, covered externally by a delicate layer of Dexter’s cells connected with its pia 
mater, from which large vessels dip vertically through its substance and enter the 
subjacent cortex. 
In what manner does this medullated band terminate 1 It has long been known 
that in Man and the higher Mammafe the external olfactory medulla ends in the tem¬ 
poral and occipital lobes, but in these animals it is seen by the naked eye that a rapid 
attenuation is undergone ere it reaches the Sylvian furrow (Plate 49, figs. 1, 2, M). 
To comprehend its distribution we must examine closely vertical and horizontal sections 
through this region anterior to the Sylvian depression. In vertical sections the super¬ 
ficial olfactory fasciculus forms the inner border of what we have termed the external 
olfactory region, limited outwards by the limbic sulcus. The cortex of this region 
is composed, as already stated, of the three layers—the peripheral zone; the irregular 
and angular cells; the large pyramidal layer. Now beneath the lowest layer of this 
cortex, at variable depths, is found an important formation of spindle-cells, which 
taking the reclinate position as regards the surface, descends from the extra-limbic 
portion of the hemisphere. This belt of spindle-cells is the representative of the 
claustral formation of higher Mammals, and is directed towards the surface of the 
cortex below, along the inner margin of the superficial olfactory fasciculus. It forms 
therefore a deep belt, dividing off the olfactory area from the external olfactory regions 
(Plate 49, fig. 8, c). Having noted this fact, a horizontal section should be taken 
through the hemisphere, close to its basal cortex, as was done for the examination of 
the connexion of the olfactory lobe with the olfactory area. If such a section pass 
through the deeper part of the superficial olfactory band we shall note that whilst its 
outer fibres are long and continued uninterruptedly backwards, the innermost fasciculi 
constantly tend to bend outwards aSiid pass through and across the longitudinal band, 
arching in fan-like manner over the whole external olfactory region as far as the limbic 
