1881.] Comparative Structure of the Brain in Rodents. 17 



certain regions. These cells are largest and most richly grouped 

 along the upper limbic arc and the sagittal border, the layer here 

 attaining a depth equal to that of the similar formation in the pig. 

 These confluent groups are so rich in cells as to exhibit from eighty to 

 one hundred in the quarter-inch field of the microscope, whilst the 

 solitary arrangement will not show more than six or eight such cells 

 in the same area. These cells are of elongate pyramidal form, the 

 largest measuring 32/t X 18/*, but they maintain great uniformity im 

 size. 



2. Modified Upper Limhic Type. — Area. Commencing near the pos- 

 terior border of the callosal commissure, this cortex spreads over the 

 median aspect of the hemisphere as far as the occipital pole and the 

 junction betwixt upper and lower limbic arcs, and also outwards over 

 the exposed aspect of the hemisphere, ending abruptly at the primary 

 parietal sulcus. — Type. It is four-laminated, the small pyramidal 

 cells of the former type gradually thinning off and disappearing 

 ultimately to be replaced by a deep belt of granule cells disposed in 

 horizontal layers separated by alternate bands of arcuate medulla. 



3. Outer Olfactory Type. — Area. Spreads over the anterior limbic 

 arc betwixt the limbic sulcus aud the superficial olfactory fasciculus. 

 Thence it covers the whole of the gyrus hippocampi, except a limited 

 area at the occipital end of the lower limbic arc. — Type. This is a 

 three- laminated cortex, possessing only two layers of nerve-cells. It 

 is constituted of — 1. A peripheral cortical zone. 2. A belt of densely 

 compressed irregular small pyramidal cells. 3. A layer of large 

 pyramids. 



4. Inner Olfactory Type. — Area. This cortex covers the " olfactory 

 field" of Gratioiet, i.e., the anterior perforated space. It is limited 

 externally by the "outer root" of the olfactory lobe, and extends 

 inwards to the median aspect of the hemisphere. — Type. A three- 

 laminated cortex, consisting of — 1. A peripheral cortical zone. 2. A 

 peculiar wavy layer of granule cells. 3. Large spindle cells, with an 

 arcuate medulla apparently connected with the claustral formation 

 externally. The spindle cells attain unusual magnitude. 



5. Modified Lower Limhic Tiype. — Area. Covers the extreme terminal 

 portion of the limbic arc, and is bounded externally by the limbic sulcus 

 and internally by the vanishing granule formation of the upper limbic 

 cortex. — Type. A five-laminated cortex especially distinguished by 

 its second layer of nerve-cells which are remarkably large, swollen, 

 and irregular, and in fact the largest elements in the whole hemi- 

 sphere. They strongly suggest in tbeir form and branching an un- 

 usually rich development of the angular cell of the second layer in 

 human brain, the elements of which, if greatly increased in size, 

 would closely resemble these cells. This cortex also includes two 

 notable arcuate bands of medulla. 



VOL. XXXIII. C 



