94 ESSENTIALS OF ZOOLOGY 



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 iris 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. 



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 cells of the eye which pass 

 the stimuli received on to the optic nerve. 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. 



The ear of the bullfrog is covered externally by a membrane, the 

 tympanum. A Eustachian tuhe runs between the middle ear and 

 mouth cavity. The tympanum has attached to it a bony rod, the 

 columella, the other end of which is joined to a portion of the inner ear. 



The inner ear lies in a cavity of the skull known as the auditory 

 capsule. The structures of the inner ear compose a memhranous 

 labyrinth which is surrounded by a lymphlike fluid, the perilymph. 

 The labyrinth is formed of a dorsal utriculus concerned with equilib- 

 rium and a ventral saccidus functioning as an auditory organ. The 

 utriculus is connected with three semicircular canals which are placed 

 in planes almost at right angles to one another. Two are vertical 

 canals, and the third, on the outer side of the utriculus, is hori- 

 zontal. The sacculus is irregular, pouchlike, and filled with a fluid, 

 the endolymph. It also contains the nerve endings which receive the 

 stimuli and convey them to the auditory nerve. 



