THE NERVOUS SYSTEM OF THE BIRD 191 
the subject of the operation. One explanation of this difference is that the resulting affection of 
sight after injury to an optic lobe depends on the extent of the lesion. If the lesion is very severe, 
involving the nuclear masses of the optic vesicle, then the animal presents marked deficiency of 
sight in the opposite eye. In a comparatively slight lesion, when the connection between the two 
optic vesicles is maintained to some extent, the animal is still capable of seeing. The question of 
the relation of the pallial tract to vision is important ; we have seen that a superficial brain lesion 
produces symptoms similar to one involving the posterior part of the brain, and by the following 
experiment Munzer and Wiener seem to have demonstrated the importance of the tract in vision. 
In a newly-fledged Bird the left hemisphere was removed, the Bird apparently seeing clearly 
with the right eye. 
In six weeks the right hemisphere was removed, but the mesial surface left intact. The 
left eye now became blind, the right remained capable of seeing, and this integrity of vision could 
alone depend on the remaining pallium. 
Has the pallial tract any other connection with vision ? 
Ferrier long ago described, from stimulating the mesial aspect of the hemisphere, 
contraction of the opposite pupil, a result which we have fully confirmed. Such stimulation, as 
the position indicated in Fig. 51, Plate X, shows, is applied directly to the superficial fibres of the 
tractus septo-mesencephalicus, and we found a certain number of degenerated fibres in the tract 
after removal of the corresponding part of the cortex, but we cannot trace any connection with 
the oculometer nuclei nor any decussation in the mesencephalon.* The vast importance of the 
mesencephalon becomes apparent if we enumerate its various connections with the rest of the 
centre nervous system : — 
1. With the corpus striatum of the same side by the tr. mesencephalicus striatus and 
tractus striomesencephalicus, which latter affords an indirect connection with the thalamus. 
2. With the pallium of the same side by means of the tr. septo-mesencephalicus. 
3. With its fellow of the opposite side by the posterior commissure. 
4. With the medulla by internuncial fibres. 
5. With the cerebellum by the tr. tecto-cerebellaris, and by the superior peduncle of the 
cerebellum. 
6. With the spinal cord by the efferent mesencephalic tracts, and by fibres in the posterior 
longitudinal bundle. 
7. The peculiar retinal connection by the median optic bundle (Perlia, Wallenberg). 
In Cats no motor symptoms other than the occurrence of forced movement of rotation are 
noticed after lesion of the corpora quadrigemina. In Birds the mesencephalic pyramidal system is 
functionally more developed, and we have been able to satisfy ourselves that after a lesion of 
moderate severity to one optic lobe, a certain amount of weakness in the opposite limbs occurs, so 
that the animals tend to fall to that side. 
* Wesley Mills, in a communication to the Royal Society of Canaila, 1898, does not agree with these observations. 
