THE NERVOUS SYSTEM OF THE BIRD 187 
Russell,* in the case of the Dog and Monkey, after severing Deiter's nucleus from its 
connection with the medulla, has found a degenerated band of fibres almost in an identical position 
to that now described by us. Many of the fibres enter the posterior longitudinal bundles of both 
sides, where they pursue an ascending and descending course. The descending fibres can be 
traced into the cord as far as the thoracic region. The tracts thus described in the Bird and 
Mammal therefore resemble each other in their course and destination. 
Russell has also described efferent lateral tracts resulting from a lesion of the medulla. 
We agree with this author in maintaining that they belong to the same system as the efferent 
mesencephalic fibres. That Russell's tract is an uncrossed one is easily explained by the view 
that the decussation has occurred higher up. 
Throughout the medulla and pons uncrossed internuncial fibres exist, entering the formatio 
reticularis and receiving additions from various levels. In some of our experiments the cerebellum 
and restiform body were involved, and sections (Figs. 46, 47, Plate IX) show that there results a 
well-marked degeneration in fibres which occupy the periphery and ventral aspect of the medulla — 
many of them cross to the opposite side, others end in the formatio reticularis or olive of the same 
side. 
The position assumed by these fibres is identical with that described by Russell as resulting 
from ablation of the middle lobe of the cerebellum, and after section of the restiform body. 
Friedlander states that in the Bird, after lesions of the cerebellum, a degenerated tract of fibres 
can be traced into the spinal cord as far as the lumbar region. Our observations are opposed to 
this view ; lesions limited to the cerebellum do not in our experience give rise to degenerations 
descending into the spinal cord. 
VI. Ascending Tracts after Lesion of the Mesencephalon 
I. Median Optic Bundle (Perlia) 
In 1889 PerliaI enucleated the eyeball of a Chicken. In addition to the resulting 
atrophy of the opposite tract, he described as the median optic bundle an atrophied strand of fibres 
which lie just median to the optic tract, and which, traced upwards, are found to terminate in the 
ganglion isthmi in the roof of the middle brain. This ganglion he consequently regarded as a 
reflex centre between the retina and the sphincter iridis. In lesions ot the corpus bigeminum we 
have found a bundle of fibres occupying identically the same position as described by Perlia. 
This degenerated bundle makes its appearance at the level where the IVth nerve becomes 
superficial, at the angle formed by the pons, cerebellum, and optic vesicle. 
It is situate just internal to the most dorsal part of the optic tract, and can be traced 
ventrally into the thalamic region as far as the foremost part of tlie optic tract. It thus contains 
fibres which degenerate in both an ascending and descending direction. This fact was demon- 
strated by us at the meeting of the Neurological Society in June, 1897, and has quite recently 
been made the subject of a special paper by Wallenberg,! who succeeded in injuring the 
' Brain,' part Ixxx, p. 427, 1898. 
f Perlia, loc, cil. 
X Loc. cit. 
