OF THE EYE MUSCLES IN MARSUPIALS. 301 



Tlie occurrence of large anterior head-cavities followed by 

 corresponding- structures behind them, has led to much discussion 

 on the metamerism of the vertebrate head. As one advances 

 in the scale of vertebrates, however, the segmentation in the 

 mesodei'in of the preotic region of the head becomes more and 

 more obscure, until in the Mammalia only traces remain of a 

 former metameric condition. No definite head-cavities have 

 ever been observed in mammals, although probable remnants of 

 these have been found in man by Zimmermann ('98). In the 

 rabbit, although the first head somite arises much as in reptiles 

 from the fore-end of the alimentary canal as a solid outgrowth 

 (Corning '99), or as a hollow one soon becoming solid (Edgeworth 

 '11), it never acquires a cavity as in lower vertebrates but in 

 later stages forms a small solid mass of cells behind the optic 

 cup on each side, difficult to distinguish from the surrounding 

 tissue. Renter ('97), who has studied the pig, sees there no signs 

 of primitive segments in the head and considers the eye muscles 

 as a completely independent formation arising from an accumu- 

 lation of mesenchyme cells ; his observations, however, begin at 

 a late stage and no early embryos are described. 



It was first noted by Milnes Marshall ('81) in Elasmobranchs 

 and is now well established, that the walls of the premandibular 

 cavity give rise to four muscles of the eye, the mm. recti superior, 

 inferior and internus and the m. obliquus inferior, all innervated 

 by the oculomotor nerve; that the second head somite contributes 

 the m. obliquus superior supplied by the trochlear nerve, while 

 the third furnishes the m. rectus externus and, in reptiles and 

 mammals, the m. retractor bulbi, both these muscles being supplied 

 by the abducens nerve. We have an exception to the usual 

 conditions in Petromyzon^ where the innervation of the muscles 

 is peculiar. According to Fiirbringer ('75, p. 70), " Die Inner- 

 vation der Augenmuskeln anlangend besteht bei den Petromy- 

 zonten die Eigenthiimlichkeit, dass, wahrend der Oculomotorius 

 der Fische eammtliche Augenmuskehr mit Ausschluss des Rect. 

 ext. und Obliq. sup. versorgt, hier der Rect. inf. nicht vom Oculo- 

 motorius, sondern vom Abducens innervirt wird, so dass also der 

 Abducens 2 Augenmuskeln versorgt. Dieses Yerhalten ist 

 vielleicht dahin zu deuten, dass der Rect. int. den vereinigten 

 Rect. int. und Rect. inf. entspricht und dass in gleichem Masse, 

 wie diese beiden Muskeln sich spaterhin in 2 gesondert, der 

 Rect. inf. mit dem Rect. ext. vei-schmolzen, einen einzigen durch 

 den Abducens innervirten Muskel vorstellend." 



The conditions in Amphibia also do not seem to coincide witli 

 those in other groups and the views of different authors are very 

 conflicting. Mai-cus ('10) has studied the Gymnophiona in some 

 detail, and according to his account of Hyijogeoiihisthe mandibular 

 somite, at first connected by a transverse bi-idge with its fellow 

 of the opposite side, develops into a large cavity surrounded by a 

 single layer of epithelial cells lying postero-dorsally to the eye, 

 and from which a cell-strand grows down into the mandibular 



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