124 J. B. JOHNSTON, 
the ventral contour of the mammillare, and is distributed to the 
several saccus tubes. The impregnation near the ventral surface 
of the hypophysis is imperfect. In Phot. 62 which is similar to 
those just described the endings of these fibres are better shown, 
while Phot. 61 is meant to show the details of the endings among 
the epithelial cells of one of the tubes. The tube at the right 
which shows several endings of fibres is separated from others by 
a capillary which is darkened by precipitate. I have found these 
efferent fibres also in methylene blue preparations of Amiurus, and 
in both Amiurus and Acipenser have traced them toward the corpus 
geniculatum. The bundle as a whole is lost near the corpus geni- 
culatum among the fibres of the postoptic decusstion. Owing to 
unfortunate breaking of my best GOLGI sections in cutting, | have 
not traced individual fibres beyond this point. 
D. Fibre Tracts of the Mid and ’tween Brain (Pl. 13). 
In describing the grey masses of these parts of the brain I 
have mentioned the fibre tracts which arise in the several nuclei. 
These fibre tracts are so complicated in their arrangement, however, 
and they make up so large a part of the brain substance that they 
require a separate description. Moreover it is upon a knowledge of 
the course and connections of the fibre tracts that our understanding 
of the functional activity of the grey nuclei must rest. I shall 
therefore attempt to describe these tracts in such detail that they 
can each be recognized in every part of their course and compared 
with the corresponding tracts in other Vertebrates. 
a) Tracts of Tectum opticum. 
1) Tractus opticus. — The optic nerve has an S-shape in 
cross section (Phot. 24). Upon entering the ventral wall of the 
thalamus all of its fibres decussate with those of the opposite nerve. 
A cross section of the chiasma (sagittal section of the whole brain) 
has a retort form. As the tract runs caudo-dorsally over the lateral 
wall of the thalamus a small part of its fibres break up in the 
nucleus anterior. Passing on, the main body of the tract enters the 
tectum at its cephalic border and runs lengthwise of the tectum 
near the mid-dorsal line. From this main longitudinal bundle fibres 
go off to break up in the outer one-third of the tectum. Here the 
fibres end in relation with the dendrites of the cells A, B and E, 
of which the cells A are much the most important. It is possible 
