ORIGIN AND STRUCTURE OF THE HEAD gt 



this view, we still see that the "head-prolongations" 

 (Kopffortsatz) of Amphioxus also produce muscles and, as 

 we have seen, the-e can be no question about considering^ 

 the latter as a distinct pair of somites, which would lie in 

 fiont of the neuiopore, i.e. the old mouth of the Annelids. 



Nature of the trigeminus. — Opinions differ again as to 

 the double nature of the trigeminus which must be assumed 

 to furnish a dorsal root to both the praemandibular and 

 the mandibular segment. According to VAN Wyhe(1882) we 

 must distinguish the ramus ophthalmicus profundus, iogtXhQv 

 with the ganglion ciliare, from the rest of the trigeminus, 

 together with the ganglion Gasseri, these being the dorsal 

 roots respectively of the praemandibular and of the mandibu- 

 lar segment. This view, suggested already, though with some 

 reserve, by Gegenbaur (1872, p. 290), has found a general 

 acceptance. ZlEGLER also, as we have seen, accepts it. Truly 

 the ramus ophthalmicus profundus could be considered only 

 as a very incomplete segmental nerve, which has lost 

 its typical branches and which, in contrast to the other 

 head nerves, has no relation to any gill-slit (as the trige- 

 minus 11 has to the mouth). Both ganglia, moreover, arise 

 in close connection with each other and at their first 

 appearance the double nature is as a rule far from obvious. 

 In higher Vertebrates there is little doubt that the insig- 

 nificant ciliary ganglion, situated over the eye, is nothing 

 but a portion detached from the ganglion Gasseri, but it 

 is questionable, if here it may be compared to the ciliary 

 ganglion of Anamnia. 



Balfour (1878, p. 214), pointing to the striking simi- 

 larity in the arrangement of the branches of the trigeminus 

 to the facialis, writes: "I was at first inclined to regard 

 the anterior branch of the fifth (ophthalmic) as representing 

 a separate nerve, and was supported in this view by its 

 relation to the most anterior of the head-cavities, but the 

 unexpected discovery of an exactly similar branch in the 

 seventh nerve has induced me to modify this view, and 

 I am now constrained to view the fifth as a single nerve, 

 whose branches exactly correspond with those of the seventh**. 

 In the same way GEGENBAUR, who was originally (1872, p. 

 290) inclined to consider the trigeminus as a double 

 nerve, though he remarks : "doch darf auch die Annahme 

 einer machtigeren Entfaltung eines einzigen Spinalnerven 

 nicht ausgeschlossen werden", afterwards (1887, p, 51) 



