The Frog 



45 



appears to form a continuation. In birds, for example, this membrane is 

 also very thin and can be thrown over the eye from the inner angle of the 

 orbit. Behind the eye is a more or less circular area called the tympanic 

 membrane, ( ) which covers the ear drum. There 



is a slight prominence in the center of this membrane produced by o?te 

 of the small bones called the columella ( ). This 



bone connects with the inner ear, and, when any sound-wave strikes the 

 tympanic membrane, the vibrations are communicated through this bone 

 into the internal ear. This gives rise to the sensation of hearing. On 

 the inner side of the tympanic membrane we find a little cavity known 

 as the Eustachian tube ( ), which opens internally 



into the mouth. There is no external ear as in man. 



The two openings immediately 

 above and behind the tip of the nose 

 are called nostrils or external nares. 

 Sometimes in front of the eyes there 

 is a little light area known as the 

 brow-spot. This was connected 

 with the brain in the embryo 

 ( ). The brow-spot 



is a feature of considerable interest 

 from the fact that in the embryonic 

 development of the frog, it connects 

 with a peculiar outgrowth of the 

 brain known as the epiphysis 

 ( ) or pineal gland. 



This is supposed to be a rudiment 

 ( ) of a stalk 



which formerly connected with the 

 medial eye ( ) 



which still persists in certain forms 

 of reptiles (Fig. 5) ( ). 



The nostrils are guarded by valves 

 which open and close during respi- 

 ration. 



The mouth extends from one 

 side of the head to the other, and the 

 anus ( ) is situated 



) end of the body. The fore limbs 

 are divided into an upper arm, a fore arm, and a manus, ( ) 



or hand, the latter possessing four digits and the so-called thumb, a 

 rudiment of the fifth. In the male the inner digit is thicker than the 

 corresponding one of the female, especially during the breeding season. 

 The entire fore arm is also relatively thicker in the male than in the 

 female. 



The hind limbs are well adapted for jumping and swimming These 



Fig. 



Mfti< 



4. Examples of Nictitating Membranes. 

 (From various authors.) 



at the posterior ( 



