CLASS AMPHIBIA 



497 



medium (im), and three car pals (a, b, c). The hand is supported 

 by five proximal metacarpal bones, followed in digits II and III 

 by two phalanges, and in digits IV and V by three phalanges. 



The hind limbs (Fig. 420, B) consist of (1) a femur or thigh 

 bone, (2) a tibio-fibula (the tibia and fibula fused) or leg bone, 

 (3) four tarsal bones, — the astragalus (tibiale, a), the calcaneum 



Fig. 420. — Skeleton of the limbs of the frog. A, fore limb, a, b, c, carpals ; 



im, intermedium ; r, radiale ; ru, radis-ulna ; I"V, digits. B, hind limb. 



a, ostragalus ; c, calcaneum ; I-V, digits ; X, accessory digit. (From Bourne, 

 after Ecker.) 



(fibular -e, b), and two smaller bones, — (4) the four metatarsals of 

 the foot, (5) the phalanges of the digits, and (6) the prehallux 

 (X) of the accessory digit. 



The Muscular System (Fig. 421). — Muscles are usually 

 attached by one or both ends to bones either directly or by means 

 of a tendon, which is an inelastic band of connective tissue. The 

 two ends of a muscle are designated by different terms : the origin 

 is the end attached to a relatively immovable part; the insertion 

 is the movable end. A muscle which bends one part upon 



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