204 ANATOMY OF THE RABBIT 



respectively anterodorsal and posteroventral. The former, in con- 

 junction with the related surface of the radius, continues the area 

 of origin of the extensor muscles of the hand from the lateral 

 epicondyle of the humerus distad on to the forearm, while the 

 latter has a similar function with respect to the flexor muscles. 

 The proximal portion of the bone is laterally compressed. It bears 

 a crescentic depression, the semilunar notch (incisura semilunaris), 

 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 pro- 

 jecting portion of the elbow, the olecranon, which is a strong 

 process for the insertion of the extensor muscles (anconaei) acting 

 on the forearm and provides leverage for their action. 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 inter- 

 osseous ligament, the radius and ulna are prevented from changing 

 their positions with respect to each other; in other words, the 

 radius is unable to rotate on an axis formed by the ulna as it does 

 in man, the forefoot being fixed in a position comparable to that 

 of pronation in the human hand (cf. p. 70). 



The Carpus 



The carpus (Fig. 94) 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 



