No. I.] AMPHIBIAN BRAIN STUDIES. 69 
and extends forwards beyond the level of the cerebellum. At 
this upper level, Fig. 18, the motor nucleus extends towards the 
centre, and sends fibres directly into the nerve root. (4) The 
descending tract is a large bundle from the more central 
region of the medulla. It rises obliquely, 5 4,4 from the root, 
Figs. 17 and 21, marked by its large, darkly stained axis cylin- 
ders, and is joined by another tract of similar fibres, the origin 
of which I have not observed, Fig. 21. The joint tract, thus 
formed, is very conspicuous and is followed without much diffi- 
culty. Opposite the cerebellum it splits into two bundles (see 
Figs. 23 and 24,5 7*). One of these passes into the cerebellum, 
Fig. 19, and, without crossing, enters the roof of the optic lobe 
at one side of the median line. The second bundle, 5 ¢4’, passes 
forwards, and scatters into rays over the whole wall of the optic 
lobe, nearly as far forwards as the posterior commissure. These 
two bundles undoubtedly arise in the same manner from the 
large cells which constitute the nucleus. 
The trigeminal mesencephalic nucleus has a re- 
markable extent in /Vecturus, reaching in two large masses of 
solidly packed cells, on either side of the median line of the 
roof, from the cerebellum to the posterior commissure. The 
posterior portion of this nucleus is shown, in Fig. 19, in which 
the cells lie, two or three deep, above the central gray of the 
ventricle. The cells are multipolar and spindle-shaped, Fig. 
19 a, with large nuclei, the main processes being directed up- 
wards into the tectum opticum. The actual connection of these 
processes with the cerebellar bundle of the descending trigemi- 
nal can be observed. It is also probable that these fibres are 
reinforced from other cells of the roof of the optic lobes, a point 
which is discussed later. This trigeminal nucleus is found in all 
the Amphibia, but is much larger in WVectwrus than in the other 
Urodela, and larger in the latter than in the Anzura. It does 
not, therefore, appear to be directly correlated with the power 
of sight, although it appears to have an indirect connection with 
the mesencephalic roots of the optic nerve. 
(5) In horizontal sections a large portion, 5 7°, of the Tri- 
geminus is observed passing forwards into the lateral portion of 
the medulla, but cannot be followed any great distance, owing 
to the similarity of its fibres with those of the main sensory 
