184 THE FROG CHAP. 



by the choroid and lined by the retina. Into the outer 

 side of this dark chamber is let a transparent window, 

 the cornea ; behind which, and separated from it by a 

 space containing the aqueous humour, is a vertical 

 curtain, the iris, perforated by an aperture, the pupil. 

 Behind the iris and in close contact with it is the lens, 

 and filling the whole of the dark chamber between 

 the lens and iris and the retina is the vitreous humour. 



The whole eye thus has the structure of a camera ob- 

 scura. The cornea, aqueous humour, lens, and vitreous 

 humour are a series of lenses, so arranged that the rays 

 of light from an external object are refracted and 

 brought to a focus on the retina, where they form a 

 greatly diminished and inverted image of the object- 

 The iris is provided with muscles, by means of which 

 the pupil can be enlarged or diminished ; it therefore 

 acts as a diaphragm and regulates the amount of light 

 entering the eye. Attached to the capsule of the lens 

 are delicate muscles, by means of which the focus of 

 the entire apparatus can be altered to some extent 

 according to whether the object viewed is nearer to or 

 farther from the eye. This arrangement for accommoda- 

 tion is, however, much less highly developed in the frog 

 than in man and the higher animals, in which the 

 relatively smaller lens is flatter and distinctly biconvex 

 in form. Thus the various parts of this complicated 

 organ are so adjusted as to bring the images of external 

 objects to an accurate focus on the back part of the 

 interior of the eyeball, i.e., on the retina. 



A vertical section of the retina (Fig. 58) reveals a 

 very complex structure. On its inner face, i.e., the 

 surface in contact with the vitreous humour, is a layer 

 of nerve-fibres (n.f), formed by the ramifications of the 

 optic nerve, which, passing through the sclerotic and 

 choroid, perforates the retina, and spreads out over its 



