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MANUAL OF ELEMENTARY ZOOLOGY 



be described. On account, however, of their larger size, they 

 are more suitable objects for the study of the eye muscles. 

 Like the eyes of the frog and those of all other vertebrate 

 animals, each of them is moved by six muscles, which arise 

 from the inner wall of the orbit. Four of these, known as 

 recti, arise together near the hinder end of the orbit and 

 diverge to be inserted into the eyeball at various points. 

 The rectus superior runs outwards and forwards and is in- 



r. int. V- °P- VU : °P- HI. 



V.md. 



Fig. 266. — The left side of the head of a dogfish with the orbit opened 

 and the eye removed. 



ltd. a., Hyoidean artery; on., orbitonasal foramen; f'sf.a., postspiracular artery; 

 o.i., 0.S., r.e., r.z., r.int., r.s., eye muscles as in Fig. 265; II.-VII., cranial 

 nerves ; ///. , third nerve entering the orbit and dividing to supply eye muscles ; 

 ///'., its branch to the inferior oblique muscle ; V.md., V.mx., V.op., mandi- 

 bular, maxillary, and ophthalmic branches of fifth nerve; Vll.hm., VII.06., 

 Vll.pal., VIl.psp., hyomandibular, ophthalmic, palatine, and prespiracular 

 branches of seventh nerve. 



serted into the upper side of the eyeball. The rectus inferior 

 runs a similar course below the eyeball to be inserted into its 

 lower surface. The rectus internus or media/is runs forwards 

 between the eyeball and the cranial wall and is inserted 

 into the front side of the former. The rectus externus or 

 lateralis runs outwards behind the eyeball, into whose hinder 

 surface it is inserted. The remaining two muscles are 

 known as obliqui. They arise together near the anterior 



