730 
MR. W. BE VAN LEWIS ON THE COMPARATIVE 
the central olfactory fasciculus, behind by the posterior olfactory fasciculus, and 
externally and deeply by the superficial olfactory medulla, and betwixt these boundaries 
the fibres pass forwards like the strands of a lyre in relation to its several sides—hence 
the term by which I have denominated this structure. 
Taenia semicircular is. —This long arciform band of medullated fibres arising from 
the summit of the gyrus hippocampi follows the curved inner surface of the caudate 
nucleus through the whole of its course, and consists of superficial and deeper-seated 
fibres, the latter connected with the ganglionic structure of the caudate nucleus. It 
has been affirmed by numerous authorities that this arciform band terminates in the 
descending pillar of the fornix.* That the more superficial fibres so terminate I will 
not deny; but I have hitherto failed in these animals to assure myself of the fact. Of 
its deeper fibres I can speak with confidence. They not only do not terminate in the 
fornix, but end in two distinct directions, the deepest curving downwards to enter the 
anterior perforated space (Plate 49, fig. 12, 2 ); the fibres above this series arch inwards 
and forwards to enter the posterior'commissure (Plate 49, fig. 12, 1 ). The latter con 
nexion is a most important one to recognise, and of its existence I have repeatedly 
assured myself by horizontal and vertical sections. The fibres enter the commissure 
from behind in such a direction as to ensure their decussation within it. The cortex of 
the gyrus hippocampi and caudate nucleus are thus brought into crossed relationship 
with the olfactory bulbs. 
2. Connexions with striate ganglion. —The anterior extremity or head of the caudate 
nucleus bends downwards to the base, and approaching the median line is separated 
from the ganglion of the opposite side by the mass of the septum lucidum. At the 
base the ganglion becomes almost superficial, being merely covered by a very thin layer 
of cortex, which is continuous with the general surface of the olfactory lobe. Medul¬ 
lated fibres from the cortex of the olfactory lobe and the granule layers of the bulb pass 
from before backwards into this basal extension of the caudate nucleus in numerous 
bundles, which in vertical sections appear mapped out into triangular wedge-shaped 
spaces by the large vessels traversing this region (pars perforata antica). This basal 
region of the corpus striatum, the olfactory field of Gratiolet, is best studied in 
fresh preparations by horizontal sections carried across the hemispheres. The brain is 
placed on the section plate of a microtome with its base downwards, frozen to the height 
of a few millimetres, the unfrozen portion removed, and then successive sections should 
be made down as far as the cortex of the base. All these sections examined fresh will 
prove most instructive. A section carried in this way through the cortex shows this 
basal or olfactory realm of the brain to be clearly mapped out into two very distinct 
areas. One, the most internal, is of pyriform contour, its narrow end continuous with 
the olfactory lobe, its larger end directed backwards. This area constitutes the base 
of the corpus striatum and has a peculiarly characteristic structure. The other, or 
outermost area, is constituted by a continuation of the olfactory cortex with the lower 
* Meckel, Arnold, Jung, and Luys. 
