CRANIAL NERVES OF SIREN LACERTINA 271 
as a distinct tract connecting what appear to be the terminal 
nuclei of the fifth nerve and in addition receiving the radix mesen- 
cephalica V. The writer is compelled to challenge the correctness 
of tne figure by Johnston, even though it is avowedly somewhat 
diagramatic. In the first place the writer is unable to verify 
the statement (Johnston ’05 a, p. 370) that in Necturus the cere- 
bellum “lies forward over the tectum opticum.”’ Kingsbury 
(95 a, plate 11, figs. 38-41) shows the cerebellar commissure in 
Necturus in the characteristic Urodele position, at the posterior 
border of the midbrain, on the middle line, decussating ventral to 
the tip of the midbrain. The writer’s preparations of adult and 
larval Necturus brains show a similar position (fig. 19). More- 
over this is the only dorsal commissure in the velar region of 
Necturus. Decussatio veli and dorsal cerebellar commissure are 
not separate structures in the Urodela. In Siren the writer 
finds at least three distinct elements in the velar commissure: 
(1) The trochlear decussation; (2) A small component from the 
radix mesencephalica V, probably from the tractus tecto-bul- 
baris et spinalis. Whether this passes across the middle line or 
not the writer is unable to say. The indications are that it 
passes to the tectum on the same side on which it enters the com- 
missure; (3) As the commissure on each side passes into the 
ventro-lateral portion of the auricular lobe (fig. 22) its fibers 
gradually vanish, presumably terminating in an end nucleus, 
possibly, as Bindewald suggests, in a terminal trigeminal nucleus. 
From the description of Osborn (’88, p. 69) the radix mesen- 
cephalica V in Cryptobranchus is very similar to that in Necturus 
and Siren. He says: 
Opposite the cerebellum it splits into two bundles. One of these 
passes into the cerebellum, and, without crossing, enters the roof of the 
optic lobe at one side of the median line. The second bundle passes 
forward, and scatters into rays over the whole wall of the optic lobe, 
nearly as far forward as the posterior commissure. 
The similar relations of the radix mesencephalica V in these 
three forms—Necturus, Cryptobranchus and Siren—make it pos- 
sible to define rather exactly the relations of the tract in the 
Urodela. From the sensory portion of the trigeminal root the 
