SEGMENTS OE TRIGEMINAL NERVE-GROUP 263 



fore-brain three and unsegmented termination, mid-brain two, and 

 hind-brain nine. 



Again, Knpffer, 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 

 ccelomic cavities in connection with them, and see whether the 

 hypothesis that such muscles represent the original dorso-ventral 

 somatic muscles of the pakeGstracan ancestor is in harmony with 

 and explains the facts of modern research. 



Eye-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, 

 i.e. 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. 



