A REPRESENTATIVE VERTEBRATE. FROG 



355 



tongue, and on the floor and roof of the 

 mouth, which probably function as organs 

 of taste. In the skin are also many sensory 

 nerve endings which receive contact, chem- 

 ical, temperature, and light stimuli. 



Olfactory organs 



The olfactory nerves (Fig. 234) extend 

 from the olfactory lobe of the brain to the 

 nasal cavities, where they are distributed to 

 the epithelial lining. The importance of the 

 sense of smell in the life of the frog is not 

 known. 



Ear 



The inner ear of the frog lies within the 

 auditory capsule and is protected by the 

 prootic (Fig. 227) and exoccipital bones. It 

 is supplied by branches of the auditory 

 nerve. There is no external ear in the frog. 

 The middle ear is a cavity which communi- 

 cates with the mouth cavity through the 

 Eustachian tube and is closed externally by 

 the eardrum (tympanic membrane). 



A rod, the columella, extends across the 

 cavity of the middle ear from the eardrum to 

 the inner ear. The vibrations of the eardrum 

 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 im- 

 pulses carried to the brain give rise to the 

 sensation of sound. The inner ears serve also 

 as organs of equilibrium. Frogs, from which 

 they are removed, cannot maintain an up- 

 right position. 



Eye 



The eyes of the frog resemble those of 

 man in general structure and function, but 

 differ in certain details. The eyeballs lie in 

 cavities (orbits) in the sides of the head. 

 They may be rotated by 6 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 transparent third eyelid or nictitat- 

 ing membrane. The lens is large and almost 

 spherical. It cannot be changed in form or 



in position, and is therefore fitted for view- 

 ing objects distinctly 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 through the pupil; and the impulses, 

 which are carried through the optic nerve 

 to the brain, give rise to sensations of sight. 



Endocrine glands 



The frog body, like that of other verte- 

 brates, is influenced tremendously by hor- 

 mones which are produced by the endocrine 

 (ductless) glands. These internal secretions 

 pass directly into the blood. Some of the 

 endocrines influence other glands, the rate 

 of growth, even the whole organism, with 

 its behavior and physiologic and structural 

 characteristics. The endocrine glands of 

 vertebrates are discussed in Chapter 33. 



Behavior 



The activities of the frog are such as to 

 enable it to exist within the confines of its 

 habitat. The ordinary movements are those 

 employed in leaping, diving, crawling, bur- 

 rowing, and maintaining an upright posi- 

 tion. These and most of its other activities 

 may be resolved into a series of inborn 

 reflexes; they are commonly said to be 

 "instinctive." Inborn reflexes result in the 

 faculty to act in such a way as to produce 

 certain ends, without foresight of the ends, 

 and without previous education in the per- 

 formance. 



Some of the movements of the frog are 

 due to internal causes, but many are re- 

 sponses to external stimuli. Frogs are sensi- 

 tive to light, and recent experiments have 

 shown that skin responses to light come by 

 the way of the eyes and pituitary gland. 

 The reaction to light causes the animal to 

 orient its body so that it faces the source 

 and is in line with the direction of the rays. 

 Nevertheless, frogs tend to congregate in 



