182 



THE FROG 



CHAP. 



of the concealed portion of the eyeball is the sclerotic (scl) 

 and is formed of cartilage in the frog ; the dark colour 

 is due to the presence of a layer of black pigment which 

 forms one of the internal coats ; this will be referred to 

 hereafter. Entering the sclerotic on its inner side, i.e., 

 the side next the brain-case, will be seen the cut end of 



C 



scZ 



Chd 



FIG. 57. The eye of the Frog. A, as seen in the living animal. B, in median 

 longitudinal section, through the plane of the optic nerve. C and D, the two 

 halves of the eye, bisected equatorially ; the inner half (C) showing the blind 

 spot, (x s.) 



aq. c. aqueous chamber ; b. s. blind spot ; c. cornea ; chd. choroid ; cj. conjunctiva ; 

 e. I. upper eyelid ; e. I', lower eyelid (nictitating membrane) ; t>. iris ; I. crystal- 

 line lens ; op. n. optic nerve ; r. retina ; scl. sclerotic; v. c. vitreous chamber. 

 (After Howes.) 



the optic nerve. The transparent, exposed portion of 

 the eyeball is the cornea (c), a superficial thin layer of 

 which, or conjunctiva (cj), is continuous with the lining of 

 the eyelids and thus with the skin covering the head ; 

 through it can be seen the coloured part of the 

 eye, or iris (ir), with a black spot really a hole in 

 its centre, the pupil. 



