I EXTERNAL CHARACTERS S 



snout are the two small nostrils, one on each side of the 

 middle line ; and, some distance behind them, the large, 

 bright, prominent eyes, in which we can distinguish, as in 

 our own eyes, a coloured ring or iris, surrounding a 

 roundish black space or pupil. The eyelids, however, are 

 rather different from our own : the upper is fairly well 

 developed, but the lower is a mere fold of skin, incapable 

 itself of being folded over the eye, but produced into a thin 

 transparent skin, the nictitating membrane, which can be 

 drawn upwards over the eye. The entire absence of eye- 

 brows and eyelashes is a point worthy of notice. 



Extending backwards from the eye is a large black patch, 

 in the middle part of which is a circular area of tightly 

 stretched skin, reminding one of the parchment of a 

 tambourine : this is the drum-membrane, or tympanic 

 membrane, a part of the ear. Here again we see a striking 

 difference from our own organs : in ourselves the drum- 

 membrane, instead of being flush with the surface of the 

 head, is placed at the inner end of a deep passage or tunnel, 

 the entrance to which is guarded by the large external ear. 

 Of the latter there is no trace in the frog. 



Attached to the trunk are two pairs of offshoots or append- 

 ages, the arms and legs, or fore- and hind-limbs, in which 

 the resemblance to our own limbs will be at once obvious. 

 The arms are very short : each consists of an upper arm, a 

 fore-arm, and a hand, the latter provided with four fingers, 

 which are slender and tapering and have no nails. The 

 legs, on the other hand, are very long : each consists of a 

 stout thigh, a long shank, with a well-marked " calf," and 

 a very curious foot. The ankle-region is long — almost like 

 a second shank — and has no heel : it is followed by five 

 toes, the first or innermost short, the second of moderate 

 length, the third longer, the fourth longer still, and the fifth 



