STRUCTURE OP THE BRAIN IN RODENTS. 
717 
Again, if our attention be directed to the variations observed from before backwards, 
i.e. } from frontal to occipital pole, over the extra-limbic realm, we first observe a 
progressive decrease in the depth of the outer cortical zone in the latter direction, with 
a decrease in size of the small pyramidal cells of the second layer. With this there is 
also associated a progressive increase of the granule or angular series which attains its 
greatest development posteriorly. On the other hand, the ganglionic series, from being 
rich in cells and deep as a stratum, at the frontal pole undergoes a progressive 
impoverishment along the regions midway betwixt the great longitudinal fissure and 
the limbic fissure, thinning out gradually to a laminar formation, whilst towards the 
extreme occipital cortex the ganglionic belt again becomes deep, with a layer of 
granules superimposed. From these particulars it may be gleaned that— 
cl The cortex near the frontal pole in the extra-limbic realm approaches the type 
of the four-laminated upper limbic cortex. 
b. The cortex midway betwixt frontal and occipital pole is a typical five-laminated 
formation, and represents, as does the six-laminated cortex of Man, the 
common type, because more extensively distributed. 
c. The cortex of the extra-limbic realm at the occipital pole is a still further 
modified form of the five-laminated, but is by no means a distinct hind of 
cortex. 
A notable diminution in size of the small pyramidal cells of the second layer occurs 
from the frontal to the occipital pole of the extra-limbic realm. Thus in front of the 
corpus callosum they average 22/xXl2/x, with a nucleus of 9 fx, and progressively 
decrease in size until, on a level with the primary parietal sulcus, they measure 
13/rX 1 1 /t (nucleus 8/x). Towards the occipital pole they again increase to 20pX 11/r. 
The same features as regards size of nerve-cell were observed along the upper limbic 
arc (l7/xXl2/x to 13pX9/x). On the other hand, the second layer in the anterior 
and loiver limbic arcs, it will be remembered, increase in size towards the occipital 
pole, wholly lose their pyramidal contour, and from measuring 19p,Xl2p, become 
globose, and measure 37/xX 32p,. 
It must be borne in mind that whilst the extra-limbic cortex appears to be developed 
out of the four-laminated cortex by the intercalation of a belt of granule cells, the 
modified upper limbic cortex is characterised by the granule cell-formation actually 
replacing the second layer of small pyramids, which is here either very defective or 
wholly absent. 
Distribution of the Ganglionic Series. 
Proceeding upon the same line of inquiry which was adopted in my former article 
upon the brain of the Pig and Sheep, it remains for me now to map out the area over 
which the ganglionic series is spread, and more especially that division of the series 
