A treatise on zoology . of thepartial polymerisation of the vagus (Hatschek [202]), according towhich the original segmental dorsal branchial nerves have beenjoined together by a longitudinal commissural epibranchial nervecoextensive with the gill-bearing region, and have lost their originalconnections Avith the spinal ganglia. In Petramyzon, indeed, the CRANIAL NERVES dorsal roots of the gill-region join the ei)ibranchial nerve. IVIeta-nieric epibranchial placodes, or proliferations of the epidermis(Beard [32], Koltzoff [272]), are found above each gill-slit; fromthese are derived the ganglia
A treatise on zoology . of thepartial polymerisation of the vagus (Hatschek [202]), according towhich the original segmental dorsal branchial nerves have beenjoined together by a longitudinal commissural epibranchial nervecoextensive with the gill-bearing region, and have lost their originalconnections Avith the spinal ganglia. In Petramyzon, indeed, the CRANIAL NERVES dorsal roots of the gill-region join the ei)ibranchial nerve. IVIeta-nieric epibranchial placodes, or proliferations of the epidermis(Beard [32], Koltzoff [272]), are found above each gill-slit; fromthese are derived the ganglia on the branchial nerves which becomejoined together by the longitudinal connection. Similar longi-tudinal nerves also unite the trigeminal witli the facial nerve, and. ^iY!)i(7u(s culiinris Risso. A, outer view of muscles and nerves of right orbit, from wluclithe eve has been removed. B, dorsal view of right eye. ar, anterior, ir, inferior, jir, posterior,and 9)- superior rectus muscle; , inferior, and , superior oblique muscle; os, cartUa-ginousoptic stalk ; 2, optic, 3, oculomotor, 4, pathetic, and 6, abducens nerve. the latter with the glossopharyngeal in Petrompon and mostGnathostomes. The exact composition and mode of formation ofthe vagus root is still far from thoroughly understood, and it ispossible that it may have been formed by a gathering together not ofwhole posterior dorsal roots as Gegenbaur suggested, but of only thosecomponents of each root which supplied the gills and alimentarycanal; this supposition (of a slipping forward, so to speak, of CRANIAL NERVES certain portions) would avoid the assumption that new centralconnections have been established with the brain (Johnston [248a]).The intestinal branch of the vagus may perh
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
