NERVOUS SYSTEM. VERTEBRATA. 129 



point of merging into the tectal commissure. The cerebellum 

 is in sagittal section, showing its cavity and the depth of 

 its transverse folding. 



D. 154. Parts of the brain of an Ostrich (Struthio camelus). The 

 anterior half of the cerebellum and the valve of Vieussens 

 have been removed, exposing the anterior part of the fourth 

 ventricle and the optic lobes. The latter are widely divari- 

 cated and are united by a broad commissural sheet (tectal 

 commissure), which is slightly thickened on the boundary 

 line between mid-brain and thalamencephalon to form the 

 posterior commissure. 



The dorsal parts of the thalamencephalon are also shown 

 by the removal of the pseudo-occipital lobes of the hemi- 

 spheres. The roof (tela choroidea) is epithelial and vascular ; 

 it is evaginated towards the anterior end to form the 

 epiphysis (somewhat damaged), and in front sends a 

 vascular offshoot into each of the lateral ventricles (on the 

 right side the hemisphere has been cut through the middle 

 of the foramen of Monro to show the continuity of this 

 plexus with the tela choroidea). The side walls of the 

 thalamencephalon are thick and appear, when seen from 

 above, as two rounded eminences (optic thalami) covered 

 laterally by the optic tracts. In the left hemisphere the 

 relatively immense proportions of the corpora striata and 

 the delicacy of the pallium are shown. 0. C. 1321 s c. 



D. 155. A cast of the cranial cavity of an Ostrich (Struthio 

 camelus) . 



D. 156. Brain of a Rhea (Rhea americana). Upon the left side 

 part of the pallium has been removed, exposing the corpus 

 striatum. A small excrescence can be seen upon the right 

 peduncle of the cerebellum, at the point towards which 

 the ends of the transverse fissures converge. This pro- 

 minence is not present in the specimens of Struthio 

 (cf. especially D. 149), but is found in most other birds ; 

 it probably corresponds with the flocculus cerebelli of 

 Mammals. O. C. 1321 R. 



VOL. n. K 



