264 Microscopie Structure of Cortical Areas 
nent position; it is present in the gyrus fornicatus where the latter 
passes into the hippocampal gyrus, and in the markedly developed ante- 
rior portion which passes from the medial to the lateral frontal surface 
of the hemisphere. This addition to the gyrus fornicatus gives to the 
brain of the horse the aspect of possessing a large frontal lobe. Such, 
however, is not the case, for in the brain of the horse the characteristic 
structure of the frontal lobe is absent. I believe we may consider this 
extensive cortical region as a part of the olfactory center. A peculiar 
feature of this region, however, is the presence of large motor pyra- 
midal cells, evidently signifying that the olfactory and motor centers 
are here merging into one another. 
Motor AREA. 
This area is mainly distinguished by the large pyramidal cells of 
Betz. These giant cells are not similar in shape or in size in different 
animals. In the horse they average about 0.44 mm. in length and 
0.374 mm. in width. In the pteropus the length of the largest cell is 
0.86 min. and its width at the base 0.18 mm. In the cat, however, these 
giant cells measure 0.612 mm. in length and about 0.468 mm. in width 
at the base of the cell. In the dog they measure 0.72 mm. by 0.324 mm. ; 
in the monkey, 0.468 mm. by 0.316 mm.; in man, 0.54 mm. by 0.36 mm. 
These large pyramidal cells of the human cortex frequently show a mass 
of pigment surrounding the nucleus, which I have not seen in pyramidal 
cells of lower animals. 
In the cortex of the pteropus, the cat, the dog, the monkey and of 
man, the motor area presents a structure of five layers. 
In the horse we can also assume five layers, but the inner stripe of 
Baillarger is here so clearly defined that it can be considered a special 
layer, making a six layer type, though such a distinction would be arbi- 
trarily based on the position of the large pyramidal cells in the upper 
portion of the stripe of Baillarger, leaving the lower portion poor in 
cells, and giving it the appearance of a distinct layer. 
In the motor area of the human cortex, there is present a suggestion 
of the granule-cell layer. This granule-cell layer does not appear 
clearly if at all in the motor area of lower animals. 
The motor cortex in the dog, the monkey and in man (less clearly so 
in the cat, horse and pteropus) is much wider than the sensory cortex 
lying behind it, but the cells in the sensory cortex lie much more closely 
together and are also much smaller. 
The ratio of thé depth of the motor cortex compared with the sensory 
cortex 1s in: 
