50 THE FROG CHAP. 



proved by the examination of a very young frog, in which 

 the single fore-arm bone is represented by two distinct 

 cartilages, the radius on the inner or thumb side, and the 

 ulna on the outer or little-finger side. The olecranon 

 is a process of the ulna. 



The skeleton of the hand is divisible into three re- 

 gions : the carpus or wrist, the metacarpus or mid-hand, 

 and the phalanges or finger-bones. The carpus consists 

 of six small irregular bones, arranged in two rows 

 (Fig. 8). The proximal row articulates with the radio- 

 ulna, while to the distal row are attached the metacarpals, 

 which together constitute the metacarpus. Four of 

 these are long, rod-like bones and support the bases of 

 the four fingers or digits : to them are attached the 

 phalanges, of which the first or innermost digit (II) 

 has two, the next two, and the remaining two digits 

 three apiece. A very small metacarpal, with a- single 

 phalanx (I), occurs on the radial side and is concealed 

 by the skin in the entire frog : it corresponds with our 

 own thumb, so that the apparent first digit of the frog 

 is really the second or index finger. 



The Hip-girdle. This, as we have seen, has some- 

 what the form of a bird's merrythought. It consists of 

 two long arms (Fig. 8, IL ; Fig. 14, //), which are 

 articulated with the transverse processes of the ninth 

 vertebra, and sweeping backwards, unite in a disc- 

 shaped mass, having on either side of it a deep, hemi- 

 spherical cavity, the acetabulum (Fig. 8, actb ; Fig. 14, G), 

 for the articulation of the thigh-bone. 



Two sutures, or lines of separation, nearly at right 

 angles to one another, divide the disc-shaped portion 

 into three parts. One of these, dorsal and anterior in 

 position, is continued into one of the arms of the hip- 

 girdle and forms half of the acetabulum ; this is the 

 ilium (Fig. 14, //, P). The second, posterior in position, 



