CLASS AMPHIBIA 505 



THE OLFACTORY ORGANS. The olfactory nerves (Fig. 423, 

 Ol) extend from the olfactory lobe of the brain (Fig. 422) to the 

 nasal cavities (Fig. 412, olf.s), where they are distributed to 

 the epithelial lining. The importance of the sense of smell in 

 the life of the frog is not known. 



THE EAR. The inner ear of the frog lies within the auditory 

 capsule and is protected by the prootic (Fig. 418, pro) and ex- 

 occipital (ex) bones. It is similar in structure to that shown 

 in Figure 350, page 411, and is supplied by branches of the 

 auditory nerve. There is no external ear in the frog. The 

 middle ear is a cavity which communicates with the mouth 

 cavity through the Eustachian tube (Fig. 411, E), and is closed 

 externally by the tympanic membrane. 



A rod, the columella, extends across the cavity of the middle 

 ear from the tympanic membrane to the inner ear. The vi- 

 brations of the tympanic membrane produced by sound waves 

 are transmitted to the inner ear through the columella. The 

 sensory end organs of the auditory nerve are stimulated by the 

 vibrations, and the impulses carried to the brain give rise to the 

 sensation of sound. The inner ears serve also as organs of 

 equilibration. Frogs from which they are removed cannot main- 

 tain an upright position. 



THE EYE. The eyes of the frog resemble those of man in 

 general structure and function (Fig. 351, pp. 411-413), but differ 

 in certain details. The eyeballs lie in cavities (orbits, Fig. 418, O) 

 in the sides of the head. They may be rotated by six muscles 

 and also pulled into the orbit. The upper eyelid does not move 

 independently. The lower eyelid consists of the lower eyelid 

 proper fused with the third eyelid or nictitating membrane. The 

 lens is large and almost spherical. It cannot be changed in 

 form nor in position, and is therefore fitted for viewing distinctly 

 objects at a certain definite distance. Movements are noted 

 much oftener than form. The amount of light that enters the 

 eye can be regulated by the contraction of the pupil. The 

 retina of the eye is stimulated by the rays of light which pass 



