TuRNEK, Morpliolooy of th<' Avian Brain. 49 



of the prosciiceplniloii to the center of the diamond -shaped depression 

 that lies immediately caudad to the twelfth nerve root. 



Dorsal fissure median is a contraction for intersection of the dorsal 

 fissure with the meson visible from above. 



Dorsal fissure frontal is a contraction for intersection of the dorsal 

 fissure with the meson not visible from above. 



A -(- affirms the presence of the fissure or tuber mentioned at the 

 top of the column. 



A — denies the presence of the fissure or tuber mentioned at the top 

 of the column. 



MESENCEl'HALON. 



The avian mesencephalon consists of two subellipsoidal 

 bodies, which lie partially or wholly beneath the prosence 

 phalon. Each lies in a special cavity of the skull. These 

 lobes have no visible connection either with each other 

 or with the diencephalon. They are usually smooth, but 

 occasionally there is a faint indicatiori of a transverse 

 fissure. 



Size. — In the same family the ratios of the dimensions of 

 the optic lobes to the length of the brain are approximately 

 constant. In different families these ratios vary. The ratio 

 of the axial length of the optic lobes to the length of the 

 brain varies from a little less than thirty per cent, in the 

 Tyrannidcr to a little more than forty per cent, in the 

 Corvidcc and Paridic. The ratio of the greatest breadth of 

 the optic lobes to the length of the brain varies from about 

 twenty per cent, to a little more than thirty per cent, (see 

 Table III). 



7\pcs. — With regard to position, the avian mesence- 

 phala fall into two classes. Those in the first class are 

 entirely covered by the prosencephalon, while those in the 

 second are only partially covered. 



When belonging to the second class the caudal portion of 

 each optic lobe lies immediately laterad to the epencephalon 

 and immediately caudad to the prosencephalon. Thence it 

 extends ventro-cephalo-mesad to beneath the prosence- 

 phalon. 



