No. 1.] AMPHIBIAN BRAIN STUDIES. vii 
magnus of Stieda. This is slightly below and in front of the 
cerebellum, 7772. 
7°. Another smaller but conspicuous nucleus in Raza is in 
the wall of the diencephalon, behind the corpus callosum, 7’, 
Fig. 29. 
The homologies of some of these nuclei are rather uncertain. 
The nucleus in the wall of the diencephalon which gives rise to 
the fibres of the optic nerve corresponds probably to the mid- 
dle or lateral geniculate body. 
The nucleus just behind the oculo-motor ganglion corresponds 
closely in position to the red nucleus of the tegmentum. 
Stieda and K6éppen have suggested that the nucleus magnus 
corresponds to the dentate nucleus of the cerebellum, but 
this is also in the position of the red nucleus and presents 
many points of agreement with it in its relations to the sur- 
rounding tracts; its relations to the cerebellum are, however, 
by no means well ascertained. 
THe Main SENSory AND Motor ENCEPHALIC TRACTS. 
There is no certain means of distinguishing the sensory from 
the motor tracts. In the ascending series of sections of the 
medulla oblongata, it has been shown that the posterior columns, 
which presumably contain unmixed sensory fibres, and the lat- 
eral columns of mixed fibres are thrust downwards by the super- 
position of the cranial nerve nuclei. As a result of this, we 
should expect to find the sensory tracts occupying the lateral 
portion and the motor tracts the median portion of the central 
region of the medulla, corresponding to the anterior columns of 
the lower levels of the cord. That such is actually the case is 
supported by two facts. First, it is found in Cryptobranchus 
that the median region consists of slightly larger and deeply 
stained fibres, more of the nature of motor fibres than those of 
the lateral region; and second, the fibres of the lateral region 
mostly terminate in the mesencephalon and diencephalon, while 
those of the median region, in large part at least, extend di- 
rectly forwards into the prosencephalon. With this evidence I 
may at all events describe these lateral tracts as sezsory and the 
median tracts as mo‘¢or. 
The Sensory Tracts. In successive sagittal sections of 
the brains of Cxyptobranchus, Necturus, and Rana, we first pass 
