SEGMENTS OF TRIGEMINAL NERVE-GROUP 263 
fore-brain three and unsegmented termination, mid-brain two, and 
hind-brain nine. 
Again, Kupffer, in his recent papers on the embryology of Ammo- 
ccetes, asserts that especial information as to the number of primitive 
segments is afforded by the appearance in the early stages of a series 
of epibranchial ganglia in connection with the cranial nerves, which 
remain permanently in the case of the vagus nerves, but disappear 
in the case of pro-otic nerves. He considers that the evidence points 
to the number of segments in the mid- and hind-brain region as 
being primitively fifteen, viz. six segments belonging to the tri- 
geminal and abducens group, three segments belonging respectively 
to the facial, auditory, and glossopharyngeal, and six to the vagus. 
From this sketch we see that the modern tendency is to make six 
segments at least out of the region of the trigeminal nerves rather 
than two. In this region, as already mentioned, the evidence of 
segmentation is based more clearly on the somatic than on the 
splanchnic segments. We ought, therefore, in the first place, to 
consider the teaching of the eye-muscles and their nerves and the 
ceelomic cavities in connection with them, and see whether the 
hypothesis that such muscles represent the original dorso-ventral 
somatic muscles of the paleostracan ancestor is in harmony with 
and explains the facts of modern research. 
EyeE-MUSCLES AND THEIR NERVES. 
The only universally recognized somatic nerves belonging to these 
segments which exist in the adult are the nerves to the eye-muscles, 
of which, according to van Wijhe, the oculomotor is the nerve of the 
1st segment, the trochlearis of the 2nd, and the abducens of the 3rd; 
while the nerves and muscles belonging to the 4th and 5th segments, 
ie. the 2nd facial and glossopharyngeal segments respectively, show 
only the merest rudiments, and do not exist in the adult. One 
significant fact appears in this statement of van Wijhe, and is 
accepted by all those who follow him, viz. that the oculomotor nerve 
has equal segmental value with the trochlearis and the abducens, 
although it supplies a number of muscles, each of which, on the face 
of it, has the same anatomical value as the superior oblique or 
external rectus. Dohrn alone, as far as I know, as already pointed 
out, insists upon the multiple character of the oculomotor nerve. 
