THE CRANIAL NERVES OP SCYLLIUM. 103 



intimate by the crossing of one of the branches of the seventh, as •will 

 be described fully later on, over the main stem of the fifth, so as to lie 

 in front of the branches of this nerve. 



The condition of the roots, as of the branches, of the fifth and 

 seventh nerves at stage o-p differs but little from that of the adult, 

 the sole change of importance as concerns the roots being that the 

 ventral roots (v /3 and vn /3) approach still closer together, and come 

 into actual contact. 



The primitive distinctness, gradual approximation, and ultimate 

 more or less complete fusion of the roots of the fifth and seventh are of 

 great interest, as proving that the fusion of these two nerves, so 

 characteristic of Pisces and Amphibia, is a purely secondary feature ; 

 and that the two are at first as independent of one another in these 

 forms as they are throughout life in the higher Vertebrates. 



In adult Elasmobranchs the combined roots of the fifth and seventh 

 nerves are usually described together, and the descriptions of different 

 observers, though not quite in harmony with one another in certain 

 details, yet agree fairly well on the main points. Stannius, whose 

 descriptions are the most precise, describes the combined fifth and 

 seventh nerves as arising in Plagiostomes by three roots, 1 of which one 

 is seen on closer examination to be double, giving four roots in all ; 

 of these the first, or most anterior one, arises from the ventral surface 

 of the medulla by two short non-ganglionic roots, which unite together 

 shortly after leaving the brain. This root is in Raja, according to 

 Stannius, mainly motor, supplying the muscles by which the respira- 

 tory movements of the anterior wall of the spiracle are effected, and 

 also certain others in connection with the jaws. The second root of 

 Stannius is large, lies posterior to the first, and is in close proximity 

 behind with the auditory nerve; it may be distinguished into an 

 anterior part which belongs to the trigeminal, and a posterior, more 

 ventrally situated portion, which belongs to the facial. The third 

 root is very large and much more dorsally situated than the others ; it 

 is connected by its deeper fibres with the second root, whilst from its 

 superficial fibres are derived, according to Stannius, the ramus ophtJial- 

 micus superficialis of the fifth, and also, in part, the maxillary and 

 buccal nerves. 



Gegenbaur, 2 in his account of the cranial nerves of Hexanchus, dis- 



1 Stannius, "Das peripherische Nervensystem der Fische." Rostock, 1849, pp. 29 and 30. 

 - " Ueber die Kopfnerven von Hexanchus," 'Jenaische Zeitschrift,' Bd. vi, 1871, pp. 

 501, 502, and 513, 514. 



