STRUCTURE OE THE BRAIN - IN' RODENTS. 
711 
For a short distance internal to the primary parietal sulcus the upper stratum still 
contains a limited number of small pyramidal cells freely interspersed with these 
granule-like bodies, and giving this upper boundary a more densely packed appearance ; 
but the lower portions of this second layer are constituted purely of granule cells. 
The latter resemble in every respect the intercalated belt of angular cells of the five- 
laminated realms of this animal, and, except in size, the smaller granules of the 
inner olfactory type and those distributed in the deep medullated structures of the 
olfactory area. 
To sum up the chief features of the modified upper limbic cortex— 
1 . It is a four-laminated cortex with a dense belt of granules for its second layer. 
2 . The granule cell formation ends abruptly at the parietal sulcus. 
3 . The upper layer possesses a deep arciform medullated band. 
4 . The granule cells are disposed in serried rank by a similar arciform stripe of 
medulla. 
The lower and anterior limbic arcs which next claim our attention comprise three 
distinct types of cortex—the outer olfactory* the inner olfactory, and the posterior or 
modified lower limbic. 
Outer Olfactory Type (Plate 50, fig. 5).* 
Vertical sections carried through the hemisphere, which pass through the septum 
lucidum, present us with four well-marked because structurally differentiated regions. 
These are, on the inner side or median aspect, the upper limbic arc j on the upper and 
outer aspect of the hemisphere the extra-limbic cortex, extending down as far as the 
limbic sulcus • beyond the sulcus* and occupying the basal aspect of the hemisphere, 
the outer olfactory portion of the lower limbic arc extends, and internal to this is a 
region continuous with the basal portion or head of the corpus striatum—the inner 
olfactory cortex.t The five-laminated cortex extending over the greater portion of 
the extra-limbic segment terminates abruptly at the limbic sulcus (Plate 49, fig. 1, C), 
immediately beneath which a wholly distinct formation commences, that of the outer 
olfactory cortex. This cortex is constituted by three layers, possessing but two belts 
of nerve-cells :— 
1 . A peripheral cortical zone (Plate 50, fig. 5, A). 
2 . A densely-compressed but shallow layer of small and very irregular pyramidal 
cells (Plate 50, fig. 5, B). 
3. A scanty series of large pyramidal cells (Plate 50, fig. 5, C). 
The outer peripheral zone has superimposed upon it, and also imbedded in its 
* Area covered by this type of cortex is shown in Plate 49, fig. 1, by cross-hatching at base, 
f The two lower of these regions are shown in diagram (Plate 49, fig. 8). 
MDCCCLXXXII. 4 Y 
