. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 202 MORPHOLOGY OF THE ORGANS OF Fig. 211. Diagram of the head segments in a selachian, after Neal. a^ anterior so- mite; aUy aortic arch; ab^ abducens nerve; dn, dorsal Tierves ; f, facialis nerve; g, glossopharyngeal nerve; gc, gill clefts; h, hypoglossal nerve; iv, intestinal branch of vagus; h^ lateralis branch of vagus; w, mediolateral line; k, neuromeres; o, otic -vesicle; oc, oculomotor nerve; op, ophthalmicus profundus Jierve \po, post-trematic nerve; j>r, pre-trematic nerve; s, spi- racular cleft; so
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. 202 MORPHOLOGY OF THE ORGANS OF Fig. 211. Diagram of the head segments in a selachian, after Neal. a^ anterior so- mite; aUy aortic arch; ab^ abducens nerve; dn, dorsal Tierves ; f, facialis nerve; g, glossopharyngeal nerve; gc, gill clefts; h, hypoglossal nerve; iv, intestinal branch of vagus; h^ lateralis branch of vagus; w, mediolateral line; k, neuromeres; o, otic -vesicle; oc, oculomotor nerve; op, ophthalmicus profundus Jierve \po, post-trematic nerve; j>r, pre-trematic nerve; s, spi- racular cleft; so, mesodermic somites; i, trigeminal nerve; â V, vagus nerve ; I-XI, neuro- meres; IâII, somites of van Wijhe; 1-7, functional gill clefts. vertebrae to the neural and branchial segments. With these as a basis he recognized nine cranial segments. Two years later Gegenbaur, using the same criteria, also concluded that there were nine segments in the head, al- though his somites and those of Hux- ley do not agree in detail. Both of these authors recognized that the nerves behind the ear (IX- XII.) were like the spinal nerves in the possession of dorsal and ventral roots, and that the ninth divides above the first gill slit into pre- and post- trematic branches (p. 63). The tenth nerve, however, bears similar relations in the ordinary sharks to four gill clefts, and hence is a compound nerve. In front of the ear the facial nerve divides above the spiracular cleft, while the trigeminal nerve splits in a similar way on either side of the angle of the mouth. This last circumstance led Huxley to the view that the mouth has arisen from the coalescence of a pair of gill slits, a view which has re- ceived a certain amount of corrobora- tion from embryology. This left a third division (ophthalmic) of the fifth nerve out of consideration ; this was supposed to represent another seg- ment further indicated according to Huxley's view by the orbito-nasal groove, while Gegenbaur saw traces of
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