262 H. W. NORRIS 
nerve may be seen emerging from the cerebellar commissure 
(fig. 18) and may be traced back toward its internal origin antero- 
ventrally along and in the inner border of the commissure to the 
point of junction of the cerebellum and midbrain (figs. 20-27). 
It is at this point that the trochlearis enters the commissure. 
From here it may be traced ventro-medially (figs 26, 27) through 
the radix mesencephalica V into the region of the longitudinal 
ventral columns. The small size of the tract and its diffuse con- 
dition render the detection of its nucleus of origin very uncertain. 
Anteriorly from its emergence from the brain the nerve runs 
along the dorsal border of the chorioid plexus (figs. 20-27, 12, 13). 
It passes from the skull in a foramen in the pars orbitalis of the 
parietal bone (figs. 10, 11). In this passage through the parietal 
bone we see an agreement with the conditions found in the Urodela 
in general (Gaupp, ’11 a), but in contrast to the condition in 
Amphiuma, where the nerve emerges between the orbito-sphenoid 
and the parietal bones. After leaving the skull the nerve passes 
anteriorly, closely pressed between the parietal bone and the 
temporal muscle, until the transverse level of the eyeball is 
reached, where it passes anteriorly, ventrally and laterally, 
through the temporal muscle, to its termination in the superior 
oblique muscle. For a short distance before reaching the oblique 
muscle the trochlearis comes into close relations with a small 
posteriorly-directed general cutaneous branch of the ramus 
ophthalmicus profundus minor V (fig. 8). In some cases the two 
nerves fuse, in other instances they are merely in contact. This 
intimate association between the trochlearis and a branch of the 
ramus ophthalmicus profundus is not uncommon in the Uro- 
dela. Miss Bowers (’00) reports it in Spelerpes, and Norris and 
Buckley (711) find a somewhat similar arrangement in Necturus. 
In Amblystoma Coghill (’02) found a close association between 
the trochlear nerve and a twig of the ramus ophthalmicus pro- 
fundus. It does not, however, appear certain that the relations 
in Siren in this respect correspond to those in these other Urodela. 
The abducens nerve arises by three or four fibers from the ven- 
tral border of the medulla oblongata, slightly posterior to the 
level of the origin of the trochlearis. Its fibers can be traced in- 
