The carpus[57] consists of six small bones arranged in two rows. The three bones of the proximal row are the ulnare, radiale and centrale. The ulnare and radiale are about equal in size and articulate regularly with the radio-ulna. The centrale is pushed out of its normal position and lies partly on the pre-axial side, partly in front of the radiale. Of the three bones of the distal row the two pre-axial ones, carpalia 1 and 2, are small; carpale 2 articulates with the second metacarpal, carpale 1 with both the first and second. The third bone is large and articulates with the third, fourth and fifth metacarpals, it represents carpalia 3-5, with probably in addition the representative of a second centrale.
The hand consists of four complete digits, and a vestigial pollex reduced to a short metacarpal.
Each of the four complete digits consists of a metacarpal and a variable number of phalanges. The first digit, as just mentioned, has no phalanges, the second and third have two, and the fourth and fifth have three.
C. The Pelvic girdle.
The pelvic girdle of the Frog is much modified from the simple or general type found in the Newt (p. 149).
It is a V-shaped structure consisting of two halves which are fused together in the middle line posteriorly, while in front they are attached to the ends of the transverse processes of the sacral vertebra. Each half bears at its posterior end a deep cup, the acetabulum, with which the head of the femur articulates.
Each half of the pelvis ossifies from two centres. The anterior and upper half of the acetabulum, and the long laterally compressed bar extending forwards to meet the sacral vertebra ossify from a single centre and are generally called the ilium; it is probable however that they represent both the ilium and pubis of mammals[58]. The posterior part of this bone meets its fellow in a median symphysis.
The posterior third of the acetabulum is formed by a small bone, the ischium, which likewise meets its fellow in a median symphysis.
The ventral portion of the pelvic girdle never ossifies, even in old animals being formed only of calcified cartilage. This is generally regarded as the pubis, but it perhaps corresponds to the acetabular bone of mammals.
D. The Posterior limb.