THE NERVOUS SYSTEM OF THE BIRD 183 
(e) Commissura Anterior. 
A large well-formed tract connecting the posterior lobes of opposite sides (figs. 8, 9, 
Plate V ; figs. 15, 16, 17, Plate VI ; fig. 40, Plate IX). It is comparatively simple as compared 
with the reptiles, but is very large, and no trace of a pars, olfactoria can be seen. Through its 
common origin with the tr, occipito-frontalis it is in connection with the anterior lobe of the brain ; 
the two combined systems thus encircling the hemisphere. Some of its fibres are seen in the 
Marchi preparations to degenerate after removal of the hemisphere. 
II. The Optic Chiasma and Tracts in connection with it 
Optic Chiasma. 
The very extensive origin of the optic tract is best demonstrated in horizontal sections 
(figs. 13 and 14, Plate V). It is seen as a large band of fibres sweeping round the external 
geniculate body to spread over the cortex of the optic vesicle. In fig. 13, Plate V, a large number 
of fibres are seen to pass from the optic chiasma immediately internal to the geniculate body and 
to terminate in the roof of the ventricle of the optic lobe. In fig. 14, Plate V, the external 
geniculate bodies are seen to be embedded in the fibres of the optic chiasma, the great mass of 
which lie on their outer side, and the smaller mass described above on tlie inner side. The 
anatomical arrangement shows the close relationship of the geniculate bodies and the optic tract. 
The central ending of the optic nerve in the bird has been described and beautifully illustrated 
by Bellonci,* with whose account of the optic chiasma this description practically agrees. 
This author also alludes to certain fibres in connection with the two bundles into which, as 
mentioned above, the optic chiasma divides ; these are : — 
(a) Inferior optic fibres, which pass from the chiasma to end in the grey matter of the 
third ventricle. 
(j8) Superior and anterior fibres, which pass through the geniculate body and unite with 
the external bundle of the optic tract, 
(y) Superior and superficial fibres lying directly above the corpus geniculatum passing 
through the grey substance of the thalamus and uniting with the external bundle 
of the optic tract. 
All these fibres are shown in our figures. 
In close association with the optic chiasma there are the following tracts : — 
1. Gudden's commissure. 
2. Commissura post chiasmatica. 
3. Commissura supra infundibularis. 
4. Epithalamic connection, 
5. Pallial connection. 
6. Connection with ganglion isthmi. 
7. Connection with the hemispheres and thalamus. 
* Bellonci, toe. cit. 
