50 JOUUNM. OF CoMPAKATlVK NeUROLOGY. 



When belonging to the first class, each lobe lies immedi- 

 ately ventrad to the diencephalon and has its major axis 

 inclined towards the meson. As in the second class, its 

 caudal end is further from the meson than its cephalic end.(') 



Excepting the Anatidic and other birds of that type, as 

 we ascend the scale, the optic lobes become itiore and more 

 covered by the prosencephalon until in the highest groups 

 there is quite a margin between the caudad extremity of the 

 prosencephalon and the caudad extremity of the optic lobe. 

 (Plate V, Figs. 9, 8,6, 13, 7.) 



This is a natural outcome of the process of cerebral revo- 

 lution, mentioned above. 



KXPLAXATION (JF TABLE III (SEE P. 82). 



"Length" is a contraction for " Ratio of the axial length of the 

 optic lobes to the length of the brain." 



" Breadth " is a contraction for " Ratio of the greatest width of the 

 optic lobe to the length of the brain." 



The length of the brain is the distance from the cephalic extremity 

 of the proseucepliiUou to the center of the rhombic depression immedi- 

 ately caudad to the twelfth nerve root. 



4" is a contraction for " Optic lobes entirely covered by the 

 prosencephalon r 



— is a contraction for " Optic lobes not entirely covered by the 

 prosencephalon y 



DIENCEPHALON.. 



This portion of the brain is, relatively, small, and is 

 entirely covered by the prosencephalon. A ventral view 

 reveals it as a small sub-rectangular body lying between the 

 prosencephalon and metencephalon. It is intimately con- 

 nected to the mesencephalon and to the metencephalon, and 

 is also connected, by the crura cerebri, to the prosencephalon. 



El'ENCEPIlALON. 



Form. — As stated above, the epencephalon is well devel- 

 oped. Its form is that of a laterally compressed hepta- 



I Prof. Cones says, that the ojilic lohes arc never coveretl ; this is evidently an over- 

 ?ht. " Cones' Key to North American Birds, 2nd edition, 1884' |). 176." 



