NERVOUS SYSTEM OF AVES. 



119 



brum, ib. a, and seems mechanically to pusli aside the optic lobes, 

 h ; the multiplied grey matter of its superficies is disposed in 

 transverse folds : small beginnings of lateral lobes are present in 

 many birds. The white neurine, fig. 45, q, continues to accumulate 

 beneath the grey, ib. d, and reduces the cavity of the originally 

 vesicular cerebellum to a fissure, ib, 7i, which retains its primitive 

 connection with the fourth ventricle, the fioor of which shows the 

 longitudinal groove called ' calamus scriptorius.' The medulla 

 oblongata expands, but its ventral surface, fig. 44, d, is not sculp- 

 tured so as to permit ' anterior pyramids,' ' olivary bodies,' a 'tra- 

 pezium,' or a ' tuber annulare,' to be defined. 



The optic lobes in the embryo, fig. 39, a, are smooth vesicles of 

 white neurine, in contact with each other, as in RejMia : they are 

 at first oblong, as in Batrachia ; next acquire a spheroid figure, 

 as in Lizards, fig. 40, b, and then assume their ornithic character 

 by diverging laterally toward the lower plane of the brain, figs. 42, 

 44, Z> : they maintain their smooth exterior, and their ventricle 

 much reduced in capacity by internal growth of neurine. 



The crura cerebri show their first superadditions, forming the 

 optic thalami, in the eight-days embryo, between a and c, fig. 39, 

 before expanding into the ' hemispheres,' ib. c. These progres- 

 sively increase in size until they acquire the relative dimensions 

 and position shown in fig. 43, a. 



They are usually of a cordiform shape with the apex directed 

 forward : in the Parrot tribe they present a more elongate, de- 

 pressed oval figure : they are devoid of convolutions ; but a 



43 



Brain of Sea-gull (Lariis). Upper view, ccxvi. 



Brain of Eayle. Base view. ccil. 



shallow longitudinal depression marks off, in some birds, a median 

 from a lateral tract of the upper surface of the hemisphere : in 

 most this surface is uniformly convex. The hemispheres present 

 an undulate surface beloAv ; the medial parts being in some birds 



