THE FROG 



membrane, ( ) which covers the ear drum. There 



is a slight prominence in the center of this membrane produced by one 

 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 present as in ourselves. 



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, which 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 respira- 

 tion. 



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 rudi- 

 ment of the fifth. In the male, the inner digit is thicker than the corre- 

 sponding 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 

 are divided into three portions, the upper portion known, however, as 

 the thigh, the middle as the cms or shank, and the distal ( ) 



portion as the foot or pes. The foot is well developed, there being five 



Fig. 4. Examples of Nictitating Membranes. 

 (From various authors.) 



at the posterior ( 



