CLASS AMPHIBTA 



529 



tive tissue sheath covering the remainder of the eyeball and known 

 as the sclera. Attached to the sclera are several muscles which move 

 the eye in various directions. The ms of the bullfrog is colorful, 

 being either golden or reddish bronze, and is clearly visible through 

 the transparent cornea. In its center is an oval opening, the pupil, 

 which can be contracted or expanded by the action of muscle fibers in 

 the iris and, like the shutter of a camera, regulates the amount of 

 light which enters the inner chambers of the eye. The lens lies 

 behind the iris and is flattened on its outer surface. It is enclosed in 

 a membrane and held in place by delicate fibers to the ciliary body. 

 The space between the cornea and lens is filled with a watery trans- 

 parent substance, the aqueous humor. 



(^^terlorverticafj^ 



Ampulla 



-"-■-^ — Sacculus 



Fig. 286. — The right internal ear of the frog, lateral view. 



The main cavity of the eye back of the lens is filled with a gelati- 

 nous tissue, the vitreous humor. The walls of this cavity are made 

 up of three layers, the outer sclerotic coat, previously mentioned, 

 then a vascular pigmented chorioid and the innermost layer, the 

 retina. The anterior portion of the chorioid forms the ciliary body, 

 which subsequently is continuous with the iris. 



The retina contains the photosensitive ceUs of the eye which pass 

 the stimuli received on to the optic ner\-e. These sensitive cells, 

 known as the rods and cones, lie embedded in the tissue so that light 

 has to pass through several layers of nerve fibers, as well as much 

 supporting tissue, before reaching them. The rods and cones com- 

 municate with fine branches of the optic nerve, which enters the eye 

 posteriorly. 



