THE FROG 71 



which is focused upon it by the lens. There are six muscles attached 

 to the eyeball by means of which it can be moved in practically any 

 direction (Fig". 466). 



THE EAR 



As has already been noted, there are really no external ears on 

 the frog though there is a rounded, flat membrane covering the real ear 

 (Fig. 19). Directly beneath this outer membrane there is another 

 tougher one which is known as the tympanic membrane ( ). 



It extends over a shallow, cone-shaped cavity called the tympanum, or 

 ear-drum, and connects with the mouth through the Eustachian tube 



( )- 



The columella ( ), a slender bar of bone and 



cartilage, extends across this, being attached to the membrane at one 

 end and connected with the inner ear at the other. It is by this little 

 bar that vibrations of the outer membrane are carried to the inner ear. 

 This inner ear is the real organ of hearing and is made up of the sensory 

 end of the auditory nerve. The auditory nerve lies embedded within the 

 skull itself. 



There are several semi-circular canals present which function as a 

 balancing organ so that the animal can keep an upright position. These 

 form what are often called an "organ of the sense of equilibrium." 



THE OLFACTORY ORGAN 



There is little known regarding what effect the sense of smell has 

 in the life of a frog, but it is known that there are little olfactory sacs 

 just within the bones into which the openings from the nostrils lead. 

 The air enters these and then passes through the bones into the mouth 

 by the internal nares. The ending of the olfactory nerve is in this little 

 sac, where it is spread out to a considerable extent and where vapors 

 of various kinds in the air may affect it. 



THE TONGUE 



The sense of taste probably resides in the tongue, though there are 

 various small structures on the roof and floor of the mouth which may 

 have similar functions. 



Conclusions of this kind are based on observations of what the fre 

 does when different tasting liquids are brought in contact with the struc- 

 tures mentioned. 



The fact that the animal does react differently to different tastes 

 is again accounted for by the finding of nerve endings in these supposed 

 taste organs. 



TOUCH AND PRESSURE 



These senses are located in the skin in various parts of the body. 

 This is due to the fact that there are many sensory nerves whose end- 

 organs terminate in the skin. Just as the experiment of the cold needla 



