FROG: EXTERNAL FEATURES AND BODY-WALL 25 



known as the upper arm or brachium, the middle segment 

 as the forearm or antebrachium, and the third segment as 

 the hand or manus. In the hand may be distinguished 

 a wrist or carpus, a palm or metacarpus, and fingers or 

 digits, of which there are only four, that which corresponds 

 to the thumb or pollex of man being absent. The first 

 finger of the male frog bears at the breeding season a 

 rough-skinned swelling, not unlike the ball of the human 

 thumb. In the hinder limb or leg, which is longer than the 

 arm, the first segment is known as the thigh or femur, the 

 second as the shank or crus, the third as the foot or pes. 

 The foot contains regions corresponding to those of the 



Fig. 8. — The palm of the right hand, A of a male frog at 

 the breeding season, B of a female, C of a male out of 

 the breeding season. 



hand, and known respectively as the ankle or tarsus, instep 

 or metatarsus, and toes or digits, but the ankle is much 

 longer than the wrist, and all five toes are present and 

 united by webs of skin, so that a wide surface is provided 

 for use in swimming. The lower side of the foot is called 

 the plantar surface, that of the hand the palmar surface. 



Between the legs at the hinder or posterior end of the 

 trunk is the cloacal opening, through which are passed the - 

 fsece_Sj.jiiine,^and--eggsTor sperm. 



The skin of the frog is a thin, tough, protective covering. 

 It contains glands of several kinds, and 

 pigment cells (Fig. 52). Between them the 

 glands provide a slimy liquid, which possesses to a slight 

 extent the acrid property found in the secretion of the skin 



Skin. 



