THE SEGMENTAL VALUE OP THE CRANIAL NERVES. 139 



inference being drawn from these special cases that the eye-muscle 

 nerves were primitively branches of the fifth nerve, which have in the 

 majority of existing vertebrates attained independence and acquired the 

 appearance of distinct nerves, a title to which they have really no claim} 



This view has very recently been advocated by Wiedersheim, 

 whom I quote in order to illustrate my statements. In dealing with 

 the fourth nerve in the frog he notices that it usually forms anastomotic 

 communications with the ophthalmic branch of the fifth nerve as it 

 crosses it, and that the number of these communicating branches is 

 very variable. He then says : — " Dies eben beschriebene Verhalten 

 sowie auch dasjenige des Abducens und des sp'ater abzuhandelnden 

 Ocidomotorius liefert eine hiibsche Illustration zu der in hoheren 

 Thiergruppen in immer starkerer Weise hervortretenden Tendenz der 

 Augenmuskelnerven, sich von ihrem Stammboden, der Trigeminus- 

 gruppe, zu emancipiren, um endlich eine gut individualisirte Selbst- 

 standigkeit zu erlangen." 2 



As this view, so definitely expressed by Wiedersheim in the above 

 passage, appears to have met with veiy general acceptance, and as it 

 very seriously affects and concerns the subject of the present paper, I 

 have taken some trouble to collect all the recorded cases in which the 

 distribution of the eye-muscle nerves or the supply of the eye-muscles 

 in vertebrates is said to present any constant deviation from the nor- 

 mal arrangement as noticed above ; and I propose now to examine 

 critically these alleged exceptions to the general rule. 



1. Amphioxus? — The azygos character of the eye and its extreme 

 simplicity of structure render any comparison with the eyes of higher 

 and more typical vertebrates perfectly futile. 



2. Marsipobranchii. 



(a) Hyperotrceti. — Among the myxinoid fishes, according to Stan- 

 nius 4 , J. Miiller, 5 Huxley, 6 Gegenbaur, 7 and others, the eye-muscle 

 nerves are completely absent. Here again we are dealing with animals 



1 Vide, e.g., Gegenbaur, " Ueber die Kopfnerven von Hexanchus," Jenaische Zeitschrift, 

 1871, pp. 548, 549 ; Huxley, Anatomy of Vertebrated Animals, 1871, p. 73; also Stieda and 

 the various authors quoted by him in his " Studien ueber das centrale Nervensystern der 

 Wirbelthiere," Zeitschrift fur wissenschaftliche Zoologie, Bd xx. 1870. 



2 Wiedersheim, in Ecker's Anatomie des Frosches, Zweite Abtheilung, 1881, p. 24, note 1. 



3 Stannius, in his Handbuch der A natomie der Wirbelthiere, Zweite Auflage, 1854, p. 161, 

 notices the absence of the eye-muscle nerves in Amphioxus. 



* Stannius, op. cit, p. 161. 



5 J. Miiller, Vergleichende Neurologic der Myxinoiden, p. 49. 



a Huxley, Vertebrates, p. 73. 



' Hexanchus, p. 549. 



