I.] I.] THE FROG. 117 



Section a. 



i. Examine high and dry and note — 



a. The eye-ball ; firm and resistant, enclosing a 



central cavity. 

 /?. The crystalline lens ; a relatively large transparent 



body filling a considerable portion of a, globular, 



its outer face somewhat flattened. It is held 



firmly in position, 

 y. The vitreous or inner chamber ; that portion of 



the cavity of the eye-ball internal to the lens. 



It is filled with a dense gelatinous vitreous 



humour, which may be raised en masse with a 



forceps. 

 S. The aqueous or outer chamber ; relatively much 



smaller than y. and external to the lens. It 



lodges a more fluid aqueous humour. 



ii. Dissect the inner portion, under water, to the level 

 of the optic nerve, so as to get the same into 

 longitudinal section. Examine the coats of the 

 eye, following their cut edges. 



a. The sclerotic; the outermost coat, dense and 

 cartilaginous ; it serves to give consistency to 

 the whole and to furnish attachment for the 

 muscles. Externally it is continued over the 

 outer face of the eye-ball as the thin and trans- 

 parent cornea ; internally it forms the sheath of 

 the optic nerve. 



/}. The choroid; internal to a, vascular and spongy 

 and blackened by pigment. It is prolonged 

 forwards in front of the lens to form the iris 

 (cf. § 1. b). 



