8 CRANIAL NERVES 



(rectus internus or anterior, rectus superior, rectus inferior, and 

 obliquus inferior). Its dorsal root is the nervus ophthalmicus 

 profundus passing to the quite anterior region of the head. The 

 ventral root of the second segment (IVth nerve or patheticus) 

 passes to the superior oblique muscle, developed from the second 

 or mandibular somite. Its dorsal root is the trigeminus nerve 

 (Vth in part) supplying the front region of the head and the jaws. 

 The myotome of the third and last prootic somite (rectus posterior 

 or externus) is innervated by a ventral root (Vlth nerve or abdu- 

 cens); and the corresponding dorsal root is the Vllth nerve or 

 facialis, which has an epibranchial branch forking over the first gill- 

 slit or spiracle, and passing down the hyoid arch. The VHIth or 

 auditory nerve also represents a portion of the dorsal root of this 

 segment. The 4th somite, first metaotic, disappears together with 

 its ventral root in all except the Cyclostomes. The glossopharyngeal 

 (IXth) nerve is the corresponding dorsal root, and supplies the second 

 gill-slit and first branchial arch. The dorsal root of the fifth segment 

 is the vagus (pneumogastric) or Xth nerve sending a branch to the 

 third gill-slit and second branchial arch. All the remaining gill-slits 

 and arches have similar epibranchial nerves which all come from 

 the vagus. Gegenbaur supposed the vagus to be a compound 

 nerve formed by the gathering together of segmental nerves equal 

 in number to the gill-slits they supply [156]. But this view, 

 that the vagus root is formed by the fusion of the dorsal roots of 

 several segments which have become incorporated into the hinder 

 region of the head, has been shown not to be in agreement with 

 embryology ; for there is no evidence of so complete a disappear- 

 ance of segments behind the first vagus root. Nor is it reconcilable 

 with the evidence of comparative anatomy. Not only are transi- 

 tory vestigial dorsal roots and ganglia found in the segments of the 

 gill -bearing region in Gnathostomes, but in the Cyclostomes 

 (Ammocoete larva of Petromyzon (Hatschek [202], Koltzoff [272])), 

 each of these segments is provided with a dorsal root, a ganglion, 

 and a dorsal branch passing to the skin between consecutive 

 myotomes. Nevertheless, it is possible that in the Cyclostome 

 one dorsal root (Hatschek [202]) and in the Gnathostome one or 

 more (van Wijhe) may coalesce with the vagus root. 



Nor do the known facts of development and anatomy support 

 the view that the distribution of the branchial nerves of the vagus 

 is due to the branching of an originally single segmental nerve. 

 There remains, as the most probable explanation, the theory of the 

 partial polymerisation of the vagus (Hatschek [202]), according to 

 which the original segmental dorsal branchial nerves have been 

 joined together by a longitudinal commissural epibranchial nerve 

 coextensive with the gill-bearing region, and have lost their original 

 connections with the spinal ganglia. In Petromyzon, indeed, the 



