186 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



made up of a pre-axial radius, fused with a post-axial ulna. The 

 boundary between these is marked by a groove at the distal end. 

 The proximal end is excavated to receive the corresponding 

 projection on the humerus, with which it forms the elbow- joint, 

 and produced back behind the last into the olecranon process, 

 belonging to the ulnar half of the bone. There are two articular 

 projections, one radial, the other ulnar, on the distal end, which 

 help to make up the wrist-joint. The endoskeleton of the manus 

 is made up of the wrist or carpus, and the bones of the digits. 

 The typical or theoretical carpus, deduced from comparison of 

 numerous cases, consists of 9 elements, 3 proximal, 1 central, and 

 5 distal, the relative position of which is as follows : [R = radial 

 or pre-axial side ; U = ulnar or post-axial side.] 



R U 



radiale intermedium ulnare 



centrale 



carpale 1 carpale 2 carpale 3 carpale 4 carpale 5 



In the frog there are six small bones in the carpus, three prox- 

 imal and three distal. Two of the former, corresponding to radiale 

 and ulnare, articulate with the radial and ulnar facets respectively. 

 The third proximal bone, the displaced centrale, is on the inner 

 (pre-axial) side of the radiale. The three distal bones correspond 

 to the carpalia. The first supports the rudimentary 1st digit 

 and = carpale 1, the second supports the 2nd digit and = carpale 2, 

 while the much larger third bone represents carpalia 3, 4, 5, fused 

 together, and supports the remaining digits. Following the wrist- 

 bones are five slender metacarpals, one to each digit. The 1st 

 metacarpal is very small, but is all that represents the 1st digit, 

 while digits 2, 3, 4, 5 are terminated by 2, 2, 3, 3 slender phalanges 

 respectively. 



(b) Hind-Limb. The hip-girdles are closely united to form the 

 pelvis, which resembles in shape a two-pronged fork with an 

 extremely short handle. Its posterior part is a rounded plate, 

 laterally compressed, and presenting on either side a deep oval 

 cup, the acetabulum, with a prominent margin. This cup is for 

 the articulation of the free limb. Nearly half of the plate and 

 acetabulum are formed antero-dorsally by the broad hinder ends 



