50 THE FROG chap. 



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 regions : 

 the carpus or wrist, the metacarpus or mid-hand, and the 

 phala/igcs 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 remain- 

 ing two digits three apiece. A very small metacarpal, with 

 a single phalanx (I), occurs on the radial side and is con- 

 cealed by the skm 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 Ave have seen, has somewhat 

 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 each side 

 of it a deep, hemispherical cavity, the acetabulum (Fig. 8, 

 acth ; 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 i\\c ilium (Fig. 14, II, P). 

 The second, |)osterior in position, is the ischium (Is) ; like 

 the ilium il is made of true bone. The third, or pubis (A'/i), 



