102 Anatomy of the; Rabbit. 



the articulating surface of which continues that of the medial 

 portion of the head of the radius, and is received into the trochlea 

 humeri. Behind the elbow-joint the bone forms the large projecting 

 portion of the elbow, the olecranon, which is a strong process for the 

 insertion of the extensor muscles (anconaei) acting on the forearm. The 

 distal extremity of the bone is formed by an epiphysis, similar to but 

 much longer than that of the radius. It is immovably articulated with 

 the radius, and its tip is formed by a blunt styloid process (processus 

 styloideus), which is articulated with the triquetral bone of the carpus. 



The elbow-joint is formed by the trochlea and capitulum of the 

 humerus in conjunction with the semilunar notch of the ulna and the 

 corresponding articular surface of the head of the radius. It is a hinge- 

 joint, or ginglymus, permitting motion in one plane, i.e., extension and 

 flexion of the forearm. The trochlear surface of the humerus, however, 

 has a slight spiral trend, the anterior portion being medial in comparison 

 with the posterior portion. 



Through the immovable articulation provided by the respective 

 proximal and distal ends of the bones, and also through the interosseous 

 ligament, the radius and ulna are unable to change their positions with 

 respect to one another; in other words, the radius is unable to rotate 

 on an axis formed by the ulna, the fore foot being fixed in a position 

 comparable to that of pronation in the human hand (cf. p. 33). 



THE CARPUS. 



The carpus (Fig. 39) comprises nine small elements, the wrist or carpal 

 bones (ossa carpi), which are interposed between the forearm and the 

 digits. They are arranged in two main rows, namely, a proximal row, 

 the elements of which are articulated with the radius and ulna ; and 

 a distal row, the elements of which are articulated with the five bones of 

 the metacarpus. Enumerated from the medial side of the wrist laterad, 

 the proximal row contains four elements, namely, the navicular, lunate, 

 triquetral and pisiform bones. The navicular and lunate are articulated 

 with the distal extremity of the radius; the triquetral with the styloid 

 process of the ulna. The pisiform bone lies on the ventral surface of 

 the extremity of the ulna, and is therefore not exposed to the dorsal 

 surface of the wrist. The distal row contains five elements, namely, the 

 greater multangular, lesser multangular, central, capitate, and hamate 

 bones. The first, second, and fourth are in association respectively with 

 the first, second, and third metacarpals. The central bone lies to the 

 lateral side of the articulation at the base of the second metacarpal. 

 As its name implies, it is originally an element interposed between the 

 proximal and distal rows. The hamate is a comparatively large element 

 associated with the fourth and fifth metacarpals, but extending also 

 to the articulation of the third, where it tends to replace the greatly 

 reduced capitate. 



