62 loiKNAK OF Com I'A I! A ri\ i: NEri:()].()(.v. 



phere to revolve towards the meson and at the same time to 

 gradually- cover the optic lohes. 



6. The avian hrain has a taxonomic value of great impor- 

 tance. vSo far, at least, as major groups are concerned, a 

 classification based upon it alone agrees in all essentials with 

 those that are based upon a careful study of all the structural 

 elements. 



III.— HISTOLOGY OF THE CEREBRUM. 



Rhiiicnccplialou (Plate VII, Figs. 6, 8, lo.) — Passing 

 entad we meet in succession the following zones: 



1. A superficial fibre zone, from the cephalic portion 

 of which projects the olfactory nerve. 



2. A gelatinous zone, in which the olfactory fibres appear 

 to become knotted. In about the middle of this zone there 

 is a narrow ring of small dense clusters of Deiter's corpuscles. 



3. A clear zone containing a few scattered Deiter's cor- 

 puscles. Near the ental portion of this zone, there is a ring 

 of nerve cells. My sections do not enable me to describe 

 with certainty the structure of these cells. They appear to 

 be of two sorts, fusiform or fiask-shaped and rhinomorphic.(' ) 

 The fusiform cells, although smaller, resemble the fusiform 

 cells of the prosencephalon. The rhinomorphic cells appear 

 to be small modified pyramidal cells. The fusiform or flask- 

 shaped cells are the prevailing type (Plate VII, Fig. 9). In 

 most of my sections I have not seen any rhinomorphic cells. 

 When they were present, they appeared to be scattered 

 among the other cells. 



4. A dense zone of Deiter's corpuscles. 



^. A row of epithelium cells lining the ventricle. Where 

 there is no olfactory ventricle this zone is necessarily absent. 



Vcjitridc (Plate VII, Fig. 5.) — In all the bird brains 

 examined, a narrow projection of the lateral ventricle extends 



I A. Bi'MM thinks that .all of the nerve cells in the olfactory lobes are iiyramidal. 

 Op. cit., p. 450. 



