THE FROG, A TYPICAL VERTEBRATE 261 



of the sympathetic system are distributed to the internal organs 

 which are thus intimately connected. 



Sense Organs. — The principal sense organs of the frog are 

 the eyes, ears, and olfactory organs. Certain structures on the 

 surface of the tongue, and on the floor and roof of the mouth, 

 probably function as organs of taste, and the many sensory 

 nerve endings in the skin receive contact, chemical, temperature, 

 and light stimuli. 



The nasal cavities (Fig. 147, o.s) are supplied by the olfactory 

 nerves (o.n) which extend from the olfactory lobe of the brain. 

 The importance of the sense of smell in the life of the frog is not 

 known. 



There is no external ear in the frog. The inner ear (Fig. 147, 

 i.e) lies within the auditory capsule, and is supplied by branches 

 of the auditory nerve. The middle ear is a cavity which com- 

 municates with the mouth cavity through the Eustachian tube, 

 and is closed externally by the tympanic membrane (t). 



The vibrations of the tympanic membrane produced by sound 

 waves are transmitted to the inner ear through a rod, the col- 

 umella. The sensory end organs of the auditory nerve are stim- 

 ulated 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 maintain an upright position. 



The eyes of the frog resemble those of man in general structure 

 and function (Fig. 147, e), but differ in certain details. The 

 eyeballs lie in cavities (orbits, Fig. 145) in the sides of the head. 

 They may be rotated by six muscles (Fig. 147) 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 al- 

 most 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 



