560 THE EYE-MUSCLKS. 



fate of this part has not however been followed. Each dorsal 

 section acquires a cup-like form, investing the posterior and inner 

 surface of the eye. The cells of its outer wall subsequently give rise 

 to three sets of muscles. The middle of these, partly also derived 

 from the inner walls of the cup, becomes the rectus iuternus of the 

 eye, the dorsal set forms the rectus superior, and the ventral the rectus 

 inferior. The obliquus inferior appears also to be in part developed 

 from the walls of this cavity. 



Marshall brings evidence to shew that the rectus externus (as 

 might be anticipated from its nerve supply) has no connection with 

 the walls of the premandibular head-cavity, and finds that it arises close 

 to the position originally occupied by the second and third cavities. 

 Marshall has not satisfactorily made out the mode of development of 

 the obliquus superior. 



The walls of the cavities, whose history has just been recorded, 

 have definite relations with the cranial nerves, an account of which 

 has already been given at p. 379. 



Head-cavities, in the main similar to those of Elasmobranchii, have 

 been found in the embryo of Petromyzon (fig. 45, he), the Newt 

 (Osborn and Scott), and various Reptilia (Parker). 



Bibliography. 



(507) G.M.Humphry. "Muscles in Vertebrate Animals." Joiirn. of Anat. and 

 Phys., Vol. VI. 1872. 



(508) J. Miiller. "Vergleichende Anatomic d. Myxinoiden." Parti. Osteologie 

 u. Myologie." ^^ad. Tf7ss., Berlin, 1834. 



(509) A. M. Marshall. "On the head cavities and associated nerves of Elasmo- 

 branchs." Quart. J. of Micr. Science, Vol. xxi. 1881. 



(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere." 

 Sitz. d. Oberhegsi^chen GeseUscliaft, 1873. 



(511) A.Schneider. Beitrfiye z. vergleich. Anat. ii. Eutwick. d. Wirbelthiere. 

 Berlin, 1879. 



Vide also Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc. 



