712 
ME. W. BEVAN LEWIS ON THE COMPARATIVE 
structure, the medullated fibres of the superficial olfactory fasciculus, which is seen in 
vertical sections as an oval or oblong belt, its fibres cut across transversely, and forming 
a notable white prominence upon the exposed surface, whilst it is also deeply imbedded 
so as apparently to rest in a slight sulcus of this cortical layer. The second layer of 
nerve-cells bends inwards in an arched form beneath it. The fibres of this superficial 
olfactory fasciculus (Plate 49, fig. 2, M) may be traced not only backwards towards 
the gyrus hippocampi, but spreading downwards into the olfactory cortex and arching 
outwards over its whole extent. The apex processes of the nerve-cells of the second and 
third layers blend with this fasciculus, whose exact connexions will be considered 
further on. The depth attained by this outer cortical zone varies between T86 mm. 
and '232 mm. In intimate structure it is fundamentally a supporting matrix of 
neuroglia, as in other regions, the nervous elements imbedded in it consisting of a 
dense meshwork formed by the branching of the apex processes of the nerve-cells 
beneath (Plate 50, fig. 5, B), and the ramifications of medullated fibres from the 
superficial olfactory fasciculus which are found at all depths of this layer beneath the 
surface. Immediately beneath the peripheral zone lies the layer of small pyramidal 
cells, forming but a shallow belt of densely-crowded elements, very irregular in 
contour, but usually pyramidal, oval, or fusiform, their apex processes bifurcating early 
and approaching in character very closely the small pyramidal cells of the second layer 
in other regions. These cells have a peculiar arrangement in clumps or appressed groups, 
especially in close proximity to the superficial olfactory tract. The nerve-cells vary 
between 18pX 10/r and 19/xX 12/x in size, and possess a nucleus whose diameter is 9 /x. 
Scattered sparsely among these smaller cells are pyramidal cells of greater dimensions, 
which become more frequent below the first belt of nerve-cells, forming here the third 
layer, through which a few really large pyramids are scattered, often measuring 
31/xX 13 ix, with a nucleus 13/x in diameter. The depth of the second layer is T39 mm. 
to T86 mm., that of the third or large pyramidal layer, varying from T06 mm. 
(olfactory region) to '568 mm. (gyri hippocampi). Tracing the passage of the extra- 
limbic formation into that of the olfactory region at the limbic sulcus, we find the 
layers of nerve-cells assume the laminated aspect peculiar to a sulcus. Thus the 
cortex becomes much shallower by the rapidly decreasing depth of the second or small 
pyramidal layer, and the close approach towards the latter of the ganglionic series. 
The second layer at the inner side of the sulcus becomes shallower and more com¬ 
pressed ; the larger pyramidal cells become sparse, form a less distinct layer, and are 
wholly absent from the innermost portion of the olfactory region, so that a clear space 
here intervenes betwixt the small pyramidal belt and the claustral formation beneath. 
Thus the second layer of the lower limbic arc is directly continuous with the small 
pyramidal layer of the cortex of the vault, whilst the large pyramids of its third layer 
are continuous with the ganglionic series of cells. The claustrum is seen beneath 
these layers descending from the parietal region as a deep stratum of spindle cells from 
•511 mm. to IT 16 mm. in depth (Plate 50, fig. 5, D). Arching beneath the limbic 
