176 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



downwards, dorsal and ventral surfaces can be distinguished. There are 

 also anterior and posterior edges, which, being in front and behind an axial 

 line running down the centre of the limb, are termed preaxial and postaxial 

 edges. (Cf. dogfish, p. 154.) 



The manus possesses four well-developed digits, and, in addition, 

 a rudimentary one concealed under the skin, and corresponding 

 to the pollex (thumb) or 1st digit of other forms. The remaining 

 fingers are 2nd, 3rd, 4th, and 5th respectively, reckoning from the 

 preaxial side (thumb side). None of the digits possess nails or 

 claws. In the male Frog a thickened pad, especially prominent 

 during the breeding-season, is found on the preaxial side of the 

 palm of the hand. It is black in II. temporaria. 



The hind-limb is also divisible into three parts (1) Femur 

 (thigh), (2) Cms (leg), (3) Pes (foot), consisting of a short cylin- 

 drical tarsus (ankle), and five slender digits, united together by a 

 delicate web. The hallux (great toe), or 1st digit, is preaxial, and 

 on its outer side there is a small, horny elevation, the " calcar," 

 which is a rudimentary " sixth toe " (prahallux). 



All the preceding regions, especially the pes, are much longer 

 than the corresponding parts of the fore-limb, but there is obvious 

 serial homology between them. The typical number of digits 

 for manus and pes in terrestrial Vertebrates is five, so that the 

 pentadactyle pes of the frog must be regarded as more typical 

 than the tetrad actyle manus. Using * for rudiment, the state of 

 things in the frog may be represented by 4 */5 *. 



Position of Body. When at rest the frog assumes a squatting 

 position, with the ventral surface near the ground posteriorly. 

 The back is humped, marking the union of the sacrum and ilia. 

 The elbow is directed backwards and outwards, and the hand 

 rests with its preaxial side turned sharply inwards. In the hind- 

 limb the knee is directed outwards and slightly forwards, while 

 the crus is bent back parallel to the femur, by which the ankle- 

 joint is thrown behind the body. The pes runs outwards and 

 forwards with its preaxial side internal. 



2. Skin (Fig. 49). The skin is thin, and raised on the dorsal 

 surface into an immense number of minute wart-like protuber- 

 ances. It is only loosely united to the body, by bands of con- 

 nective tissue along certain definite lines. The intervening 

 subcutaneous spaces, over which the skin is baggy, are filled with 

 lymph. 



