THE FROG — A REPRESENTATIVE VERTEBRATE 



415 



^rantum. 



AcotLstic 

 nerue 



Medulla 

 oblon^a.^a. 



Inner ea-r 

 (meirLbraxious labyrinth) 



Otic Capsule 

 Tympanic membrane 

 Fenestra, ovalis 



■pta-pes 



Middle ear 

 cavity 



Eustachian 

 tu.bc 



Figure 21.17. A diagrammatic cross section through the head of a frog to show 

 the ear and its relation to surrounding parts. 



for taste are gathered in taste buds located on the tongue, and in other 

 parts of the lining of the mouth and pharynx. 



The eyes of frogs are very similar in basic structure to those of 

 mammals, which are described in Chapter 29, but the method of ac- 

 commodation is different. A frog focuses on near objects by moving the 

 lens of the eye forward, thereby increasing tlie distance between the 

 lens, which is located near the front of the eye, and the light-sensitive 

 retina located at the back of the eye. The same thing is done to focus 

 a camera on near objects. In the mammalian eye, the shape of the lens 

 is changed in focusing. 



The ears receive sound vibrations which set a tympanic membrane 

 in vibration. The vibrations are transmitted across a middle ear cavity 

 by a rod-shaped bone known as the stapes (Fig. 21.17). This cavity is 

 comparable to a gill pouch of a fish, which is connected to the pharynx, 

 so it is not surprising that it is connected to the pharynx by a Eustachian 

 tube. The inner end of the stapes fits into an opening in the otic 

 capsule known as the oval window (fenestra ovalis). An inner ear, 

 consisting of a series of liquid-filled canals and sacs, lies within the 

 otic capsule. Vibrations of the stapes are transmitted to a specific group 

 of cells within the inner ear, which are stimulated and initiate impulses 

 in the acoustic nerve. By this means the vibrations are perceived as 

 sounds. Other cells in the inner ear are stimulated by the motion of the 

 liquid in the canals and sacs that is brought about by changes in the 

 position of the body. Thus the inner ear is concerned with equilibrium 

 as well as sound detection. 



191. Nervous System 



The various parts of the nervous system are commonly grouped 

 into a central nervous system, which includes the brain and spinal 



