268 Microscopic Structure of Cortical Areas 
cortex, so that it can be accepted as the rule there. In the dog and 
monkey, on the other hand, the inner layer of pyknomorphous pyra- 
midal cells is very distinctly developed and spread over a fairly large 
area. Now if, as is generally done, small pieces of the cortex are cut 
here and there from the surface of the hemispheres, it may happen that 
either the outer or the inner layer of pyknomorphous pyramidal cells 
alone is present (in most places the outer layer, as it is present almost 
everywhere in the seven-layer type), a study of such pieces might lead 
one to. believe that the inner or outer layer of pyknomorphous pyra- 
midal cells does not exist at all. In those places where the internal 
layer of pyknomorphous pyramidal cells is absent, it is difficult to recog- 
nize a seven-laver type, if by absence of these cells the stripe of Bail- 
larger is not plainly brought to view. I have not as yet been able to 
definitely fix the exact points, where these variations occur, but one can 
see that these pyknomorphous pyramidal cells are present in greater 
numbers in the dorsal than in the ventral portion of the parietal lobe. 
It is also noticeable, that the inner layer of pyknomorphous pyramidal 
cells disappears as the occipital lobe is approached, so that posteriorly 
the stripe of Baillarger comes more into view. 
The seven-layer type is found in the dog, behind the fissura cruciata, 
and extends backward to within 2 cm. of the occipital pole. It is found 
less sharply marked in the temporal lobe and is entirely absent in front 
of the fissura cruciata. 
In the monkey the seven-layer type is most distinctly marked in the 
parietal region lying between the central suleus and the external occi- 
pital fissure, or ape cleft. In the temporal lobe this type undergoes a 
modification, which will be described below. In front of the fissure 
which corresponds to one of the frontal fissures in man (Munk calling 
it “ Harkenfurche ” in the monkey), this type is again evident and its 
limits coincide with those of the frontal lobe. 
In the human cortex, this type extends, as I have stated before, over 
a very large area. It is found in the parietal lobe and in the precuneus. 
It occupies the entire lateral surface of the occipital lobe, and extends, 
modified, into the temporal lobe. 
On examining this type, the question suggests itself, what may the func- 
tion of this seven-layered cortex be? Extirpation experiments performed on 
the Monkey, especially those of Munk, seem to show that the muscular 
sense is located in the parietal lobe. But he points out two areas in the 
monkey showing this type, the destruction of which is not followed by any 
focal symptoms. Of these two cortical regions, one lies in front of the 
frontal suleus, Harkenfurche, which undoubtedly corresponds to the frontal 
lobe in man, and the other is a small area at the posterior dorsal extremity 
