140 PROFESSOR MARSHALL. 



in which the eyes are in a very rudimentary condition, and the eye- 

 muscles either absent or extremely imperfectly developed ; so that, as 

 pointed out by Schwalbe, 1 no importance can be attached to them in 

 determining the question of the primitive independence of the eye- 

 muscle nerves, and this consideration is much strengthened by the 

 strong evidence we possess of the Myxinoids being degenerate or de- 

 graded forms. 2 



(b) Hyperoartii. — Attention has been directed to the condition of the 

 eye-muscle nerves in the lampreys by a number of writers. According 

 to Schlemm and d' Alton, 3 the lampreys have independent eye-muscle 

 nerves, but their number is diminished, and some of the muscles are 

 supplied by the fifth nerve. The fourth nerve is described as having 

 its usual origin behind the optic lobes and entering the orbit in com- 

 pany with the third, which has an independent origin in front of that 

 of the fifth. The combined nerve, formed by the union of the third 

 and fourth, divides into two main branches, an upper one supplying 

 the rectus, superior, and a lower one supplying the rectus interims and 

 obliquus superior. The three other muscles, viz., rectus inferior, rectus 

 extermis, and obliquus inferior, are said to receive their nerves from 

 the trunk of the fifth nerve. 



Fischer 4 and Stieda 5 also refer to the peculiar distribution of the eye- 

 muscle nerves in the lampreys, but avowedly draw their information 

 from Schlemm and dAlton's paper, from which it would appear that 

 Huxley, 6 and probably Owen 7 and Giinther 8 also, derive their ac- 

 counts. 



Gegenbaur 9 gives a slightly different account. He says that in 

 Petromyzon there is an independent fourth nerve, but that the sixth 

 is a branch of the fifth, which supplies the rectus inferior as well as 

 the rectus extermis, while the third nerve is limited in its distribution, 

 supplying the rectus superior, rectus internus, and obliquus inferior. 

 He gives no reference in support of his statement, and must therefore 

 be supposed to make it on his own authority, especially as it differs 



1 Schwalbe, "Das Ganglion Oculomotorii, Jenaische Zeitschrift, Bel. xiii., p. 71. 

 a Cf. Balfour, Comparative Embryology, vol. ii., 1881, p. 263, note 2. 



3 Schlemm u. D'Alton, " Ueber das Nervensystem der Petromyzon," Muller's Archiv, 1838 



4 Fischer, A mphibiorum nudorum Neurologios Specimen Primum, 1S43, p. 47. 



6 Stieda, loc. cit., p. 174. 



Huxley, Anatomy of Vertebrated Animals, p. 73. 



7 Owen, Anatomy of Vertebrates, 1866, vol. i., p. 300. 



8 Giinther, Introduction to the Study of Fishes, 1880, p. 105. 



9 Gegenbaur, Hexanchus, p. 549, note 1. 



