256 THE BIOLOGY OF THE ANIMAL 



H. — Tlie Eye mid the Ea/r. 



a. The eye. — Examine the eye of a living frog, 

 and make out the transparent outer portion or 

 cornea, below which lies a pigmented ring, the 

 iris, in the centre of which is a black opening, 

 the pupil. What is the color of the iris % How 

 much of the exposed portion of the eyeball does 

 it form ? What is the shape of the pupil ? Diam- 

 eter ? Watch the eye carefully to see if the pupil 

 changes in size. KiU the frog with chloroform, 

 and remove the eyes by cutting the eyelids and 

 muscles. Note how the lids are connected to the 

 eyeball by means of the conjunctiva. Examine 

 again the shape of the eye, and find the point of 

 entrance of the optic nerve. Notice the white 

 sclerotic membrane which forms the outer 

 coat of the eye. Place the eyes in a watch-glass 

 of water, and with sharp scissors divide one by 

 a cut passing through the centre of the pupil and 

 slightly to one side of the optic nerve ; the other 

 by an incision at right angles to the line passing 

 through the pupil and the nerve. With a lens 

 study the sections first made and notice the black 

 choroid membrane, which lines the cavity or 

 vitreous chamber, in which lies the vitreous 

 humor. What is the relation of this membrane 

 to the sclerotic ? To the iris ? What is the ap- 

 pearance of the vitreous humor? Remove it and 

 examine its shape. Anterior to the vitreous hu- 

 mor find the crystalline lens. What is its 

 shape? Size as compared with the vitreous hu- 

 mor? In front of the lens is the anterior 

 chamber of the eye containing the aqueous 

 humor. Examine the sections of the other eye, 



