THE CEKEBKUM OE PKOSENCEPHALON. 



161 



There is no other part of the brain which approximates the cerebral 

 mantle in the great changes in the progression and retrogression manifested; 

 and, since this is involved in the existence of certain higher psychic activi- 

 ties, let ns now proceed with the consideration of the most interesting field 

 of brain-anatomy. 



First, as to the outer form. What has already been said regarding the 

 selachian mantle has shown that in that class of vertebrates only the anterior 

 region of the mantle is of nerve-tissue, but that larger or smaller portions — 

 according to species — of even the posterior part of the mantle have given 

 up the character of simple epithelium. 



!N'ote in Fig. 107 the thin mantle of the trout as compared with the 

 enormous thickening which the anterior portion of the mantle has under- 

 gone in the ray (Fig. 108). Then note that in the amphibia (Fig. 109) the 

 thickening has progressed much farther posterior. Further note that the 



Fig. 107. — Schematic sagittal section of an embryonal t«leostean brain (trout). 



brain of the reptile, with its already developing cortical substance (Fig. 110), 

 forms a transition to the birds, on one side, and to mammals, on the other 

 side (Figs. Ill and 112). 



In our description of the hemispheres, which we will always find in 

 amphibia and upward in the vertebrate series, it will be best to take as a 

 starting-point the ovoidal form. In amphibians and reptiles the smaller an- 

 terior end of the ovoid merges into the olfactory lobes, while, on the median 

 side, — the one turned toward the other hemisphere, — there takes place so 

 marked a flattening that only a vertical cleft remains between the two halves 

 of the brain. 



In the midst of the cleft the two halves of the brain are connected by 

 the unpaired Lamina terminalis, which passes in a convex line from above 

 downward and forward. But the hemispheres have been developed, not only 

 anteriorly from the Lamina terminalis, as is stated in the embryological in- 

 troduction. They usually extend dorsally as well as ventrally from the 



