70 OSBORN: [Vor. II. 
tract adjoining. This encephalic tract underlies the similar 
bundles from the Auditory and Facial, Fig. 18. 
Abducens, Trochlearis, and Oculo-motorius. The most inter- 
esting facts observed in connection with these nerves is the 
close relation of their nuclei to the posterior lon- 
gitudinal fasciculus and the relation of the oculo- 
motor nucleus to the posterior commissure. Both 
of these observations have been made before, but in the Am- 
phibian brain they stand out with unusual clearness. In ver- 
tical and longitudinal sections, Figs. 20 and 20 a, some of the 
large fibres of this fasciculus seem to pass directly into the 
larger processes of the cells of the oculo-motor nucleus, 3 2!. 
The larger processes seem to lie in the direction of these 
fibres; the smaller processes are partly directed towards the 
fibres of the posterior commissure, and actually extend into 
the lower portion of this commissure; they are partly directed 
towards the exit of the nerve. This group is figured with 
the camera as observed in Cryptobranchus ; the posterior com- 
missure also descends close to this nucleus in Wecturus and 
Rana, as seen in vertical and transverse sections, Fig. 25. 
The posterior longitudinal fasciculus also has close 
relations with the Abducens nucleus, as shown in Fig. 18, 
although I have not observed either in this or in the Troch- 
learis? the actual continuity of its fibres with any of the 
cell processes. The Abducens nucleus is directly opposite 
the exit of the 5th pair, but unlike all the posterior nerves 
of the medulla, it passes out close to the middle line, more in 
the manner of an anterior spinal nerve root. 
The Oculo-motor nucleus is double, consisting of two 
small groups of typical motor cells, 3 2 and 3 x’, the fibres of 
the posterior commissure descending between them. As found 
in Wecturus, Fig. 25, the fibres all arise upon the same side of 
the brain, and the nucleus is placed at the edge of the central 
gray substance. 
1 It is difficult to follow the Trochlearis from its nucleus to its exit. It is 
stated to contain sensory fibres in the Selachians and Amphibia. Gegenbaur, op. 
cit. p. 49. I have only observed a motor nucleus. 
