CRANIAL NERVES OF SIREN LACERTINA 263 
ternally into the immediate vicinity of the ventral longitudinal 
columns. After emerging from the brain the abducens passes 
antero-ventrally across the arachnoidal and sub-dural spaces into 
a small foramen in the cartilaginous base of the cranium, thence 
-antero-laterally between the basal cartilage and the parasphenoid 
bone (fig. 13). A little anterior to the level of the anterior border 
of the ear capsule it passes laterally and dorsally around to the 
lateral border of the orbito-sphenoid cartilage in the connective 
tissue between that cartilage and the quadrate (fig. 12), taking a 
position ventral to the ramus ophthalmicus profundus V as the 
latter leaves its ganglion (fig. 11). For some distance the abdu- 
cens runs anteriorly, ventral to the profundus, but at the level 
of the oculomotor foramen it has taken a position at the ventro- 
medial border of the profundus, between the latter and the orbito- 
sphenoid cartilage. This position it maintains until the rectus 
externus muscle is reached. Thus it will be seen that the ab- 
ducens nerve in Siren is completely independent of all other nerves, 
from its origin to its termination, and thus retains what may be 
considered a primitive arrangement. 
THE TRIGEMINAL NERVE 
1. The roots of the trigeminal nerve 
Three fiber tracts, or groups of rootlets enter into the composi- 
tion of the fifth cranial nerve: (1) The bulk of the fibers of the 
fifth nerve, on entering the medulla, turn posteriorly into the 
tractus spinalis trigemini (Vsp.) which may be traced as far as 
the level of the third spinal nerve. In Necturus Kingsbury 
(95 a, p. 189) finds a tract of fibers related to a nucleus of cells 
in the gray matter adjoining the spinal V tract and connected 
with the latter just after the latter enters the brain. Osborn 
(88, p. 68) recognized a similar tract in Cryptobranchus. In 
Amphiuma the writer (’08, p. 530) found a corresponding tract, 
but very doubtfully considered it distinct from the spinal V 
tract. In Siren such a tract does not appear distinct from the 
spinal V, unless it be found in the small posteriorly directed 
tract (Vrmp.) given off from the radix mesencephalica as the lat- 
ter enters the brain. (2) Motor fibers enter the trigeminal nerve 
