THE FROG 15 



brachium, a small indistinct wrist or carpus, and a hand or manus. 

 Unlike our own the hand has only four fingers or digits, a thumb or 

 pollex being absent. The leg similarly is composed of a thigh or 

 femur, a shank or cms, a very long ankle or tarsus, and a foot or 

 pes. The foot appears to have six digits, five, large and one small, 

 but we shall see later that the small inner one, the calcar, is not a 

 true toe, and the first big one is the great toe or hallux. The five 

 large toes are united by a thin fold of the skin, the so-called web, and 

 in this way a large surface for swimming is obtained. The form of 

 the animal is maintained by a number of hard structures or bones, 

 that are felt when handling it, and which together constitute the 

 internal skeleton or endoskeleton, to distinguish it from the exo- 

 skeleton. 



Among the amphibia in general it is difficult to tell the male from 

 the female externally, but this may be done quite easily in the frog 

 by an examination of the hand. The under side of the inner digit 

 in the male possesses a brownish-black swelling, somewhat like the 

 ball of our limb, but still more marked, while the same digit of the 

 female is not swollen. This enlargement in the male varies at 

 different times of the year, and is particularly well developed during 

 the breeding season. Further, at the same period the male when 

 croaking exhibits two bladder-like swellings, the vocal sacs, under 

 its throat, these serve as resonating chambers to increase the volume 

 of sound. 



General Internal Structure. 



Having considered the external characters, out next duty 

 is to examine the internal structure. If the mouth be opened 

 widely and the joint between the jaws snipped through with the 

 scissors a number of structures can be made out. The roof of the 

 mouth, or buccal cavity, is surrounded by the upper jaw, which is 

 immovably fixed to the skull, as in ourselves, and contains a large 

 number of small pointed teeth, the maxillary teeth. The roof itself 

 is formed by the base of the skull and the orbits or cavities in which 

 the eyes are lodged, and is covered by a very soft moist skin, the 

 mucous membrane. Two very prominent swellings caused by the 

 eyes are readily seen. Just in front of these, nearer the middle line, 

 are two small groups of teeth, the vomerine teeth, and in front of 

 these again two small openings, the internal nostrils or nares. These 

 communicate with the internal nares, as may be seen by inserting 

 a bristle into the latter, by means of a short tube, the narial passage. 

 This channel is dilated to form a large narial chamber containing the 

 olfactory organ, by means of which the frog is able to smell. Behind 

 and outside the eye swellings at the back of the upper jaw are two 



