108 



SUB-CLASS MARISPOBRANCHII (CYCLOSTOMATA). 



The fifth nerve divides into two branches, the ophthahnic * which is 

 purely sensory and passes to the skin on the head, and the ventral 



branch, which is both motor 

 and sensory. The ventral 

 branch divides into an ex- 

 ternal and internal branch, 

 which do not correspond to 

 the superior and inferior 

 maxillary branches of other 

 fishes, for they both supply 

 muscles which in Selachians 

 are supplied by the inferior 

 maxillary branch, f The 

 seventh nerve is in Petromy- 

 zon a purely sensory nerve ; 

 in Myxinidae it is mainly 

 sensory. 



The vagvis arises by eight 

 roots, of which the four an- 

 terior group themselves to- 

 gether as a nerve which by 

 its distribution to the tissue 

 between the first and second 

 branchial pouches must ob- 

 viously be compared to the 

 glossopharyngeal (Fig. 57, 

 Br^). The other four vagus 

 roots unite in a ganglion 

 which is joined by a com- 

 missural branch from the 

 seventh nerve {vii.-x.) and 

 gives off dorsally the lateralis 

 (lateral line branch) and ven- 

 trally the visceral branch. 

 The latter supplies the bran- 

 chial region and the whole 

 length of the intestine, in the 

 wall of which it lies. In the 

 branchial region in Petromy- 

 zon, and possibly in the intes- 

 tinal region as well, the vis- 

 ceral branch of the vagus is 

 connected with the posterior 

 roots of the spinal nerves. 

 As suggested by Mviller, it 

 very possibly represents the 

 sympathetic which is other- 

 wise absent in Marsipo- 

 branchs. 



Some ventral roots arise 

 behind the vagus (so-called ventral vagus roots) and vmite to form a nerve 



* A motor branch is mentioned by some authors. 

 f Fiirbringer, Joe. cit. 



N.:p. 



Fig. 57. — Diagrammatic dorsal view of the pos- 

 terior cranial nerves of Petrormjzon (after 

 Ahlborn). a Sensory root of hypoglossal 

 from the glossopharyngeal ; Au auditory 

 capsule; -Br, glossopharyngeal; G. sp . 1, 

 G.sp 2 etc., ganglia on postrior roots of spinal 

 nerves ; Lot. lateral branch of vagus ; N.X. 

 vagus roots (including those of the glosso- 

 pharyngeal) ; N. sp. spinal nerves ; Ophth. 

 ophthalmic branch of trigeminal ; Pn vagus ; 

 rd dorsal, r.v ventral ramus of ventral root 

 of spinal nerve ; vm motor, vs sensory root 

 of trigeminal ; iv.d. dorsal, tv.v. ventral roots 

 of spinal nerves ; VII facial nerve ; VII-X 

 branch connecting facial and vagus ; VIII 

 auditory nerve ; XII hypoglossal ; XII rd. 

 dorsal branch of XII to muscles of head . 



