I EXTERNAL CHARACTERS 5 



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 covering the eye, but produced 

 into a thin transparent skin, the nictitating membrane, 

 which can be drawn upwards over the eye. The entire 

 absence of eyebrows and eyelashes is a point worthy of 

 notice. 



Extending backwards from the eye is a large dark 

 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 

 appendages, 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 inner- 

 most short, the second of moderate length, the third 

 longer, the fourth longer still, and the fifth of about the 



