VIII. 



THE COMMON FROG. 



123 



The nerves of the frog exhibit certain intermediate 

 conditions, hke those we have seen to exist in various 

 other parts of its anatomy. 



In the higher vertebrate animals, as in J\Ian, the 

 muscles which move the eyeball are supplied by 

 three distinct nerves, termed respectively the 3rd, 

 4th, and 6th. The 5th nerve being a very large and 

 complex one, sending branches to various parts of 

 the head and its or^-ans. 



Fig. 75. — ^The Muscles of the Eyeballs of Man, viewed from above and the outer side. 

 R.S., the superior rectus : Itif.R., the inferior rectus ; E.R., the external rectus ; 

 Ift.R., the internal rectus; S.Ob., the superior oblique; Itif.Ob., the inferior 

 oblique ; Ck. the chiasma of the optic nerves (//•) ; ///., the third nerve, which 

 supplies all the muscles except the superior oblique and the external rectus. 



Now in the frog there is no distinct 6th nerve, it 

 being replaced by an extra branch of the 5th neri^e. 

 This modification, however, is but one'^tep towards a 

 condition which obtains in the Mud-fish {Lepidosireii), 

 when all these three nerves are quite blended with 

 one division (the Ophthalmic) of the 5th nerve. 



Again in the higher Vertebrates, as in Man, the 

 8th nerve is a very large and complex one, and is 



