THE FROG AND VERTEBRATES IN GENERAL 219 



of the levator bulbi muscle which lies obliquely along the ventral 

 part of the orbit. 1 The other muscles are (1) a pair, the superior 

 and interior recti, which rolls the eye up or down ; (2) a pair, the 

 anterior and posterior recti, which rolls the eye to the right or 

 left, and (3) a pair, the superior and inferior oblique muscles, 

 which gives the eye oblique movements. 



Lying within the sclerotic layer is the thin choroid layer. It 

 contains many blood vessels, is deeply pigmented, and, anteriorly, 

 forms the colored portion of the eye, or iris, in the center of which 

 is a circular opening, the pupil. The iris also contains muscular 

 elements, and in its functioning may be compared to the iris 

 diaphragm in a camera. When there is not enough light the radiat- 

 ing muscles in the iris contract, thus enlarging the pupillary open- 

 ing, through which light is admitted to the interior of the eye. In 

 bright light, the circular muscles of the iris contract and this 

 results in a constriction of the pupillary opening so that only a 

 small amount of light is admitted to the sensitive portions of the 

 eye. 



The innermost layer of the eye is the retina. This contains the 

 sensory cells which are adapted for receiving the photic stimuli and 

 passing them on to the optic nerve. The retina lines the greater 

 part of the eyeball, but does not extend to the anterior surface as 

 do the other two layers. It consists of (a) a thin outer layer con- 

 taining pigmented cells, which lies next to the choroid coat of the 

 eye and (6) a thicker sensitive layer containing the nerve tissue, 

 which forms the inner lining of most of the eye cavity. The 

 sensitive layer of the retina is an extremely complex structure, and 

 a microscopic study of properly prepared material shows that it is 

 composed of no less than nine layers of tissue, the last of which is 

 the sensory tissue composed of very highly specialized cells showing 

 two structural types, the rods and the cones. The former are 

 more numerous and appear as narrow, elongated bodies with dis- 

 tinct striations. The rods also show a differentiation into a larger 

 and a smaller type. The cones are considerably larger but not so 

 elongated as the rods. (W. fs. 150, 151.) 



The optic nerve enters the retina posteriorly. It passes through 

 the retinal layers giving off fine branches which run to all portions 

 of the retina and, finally, innervate the essential rod and cone cells. 

 The part of the retina where the optic nerve enters lacks these 



1 The two muscles just described are not present in the human eye. 



