74 



ZOOLOGY. 



see presently. Another feature of the humerus is the 

 prominent deltoid ridge (representing the region of attach- 

 ment of an important muscle) on its preaxial border. Of 

 the two bones in the next segment of the limb, the preaxial 



is called the radius, the postaxial 

 the ulna (fig. 39). The ulna has a 

 projection, the olecranon process, 

 beyond the articulation with the 

 humerus (elbow) : this gives leverage 

 to the extensor muscles (i.e. muscles 

 which straighten out the arm), 

 whose tendons are attached to it, 

 and it is to make room for this 

 process when the arm is extended 

 that the humerus has an olecranon 

 fossa. 



In the primitive position of the 

 limb, radius and ulna lie side by 

 side, but the partial rotation of the 

 fore-arm necessary to bring it into 

 the prone position causes them to 

 be slightly crossed in the rabbit. 



The small bones that follow con- 

 stitute the carpus collectively each 

 one being a carpal. The three of 

 the first row are called radiale, 

 intermedium and ulnare, in refer- 

 ence to their position with regard 

 to the bones of the fore-arm. The 

 single one of the second row is 

 called centrale; the five of the 

 distal row are called distalia and 

 numbered ; the fifth distale is, how- 

 ever, fused with the fourth. Of the 

 rod -like bones that support the 



fingers, it is customary to call the proximal in each finger 

 metacarpal, and the rest phalanges, though apart from 

 human anatomy there is no justification for such a dis- 

 tinction. The preaxial digit (pollex or thumb) has two 

 phalanges only, all the others three. 



Fig. 39. RADIUS AND ULNA ; 

 CARPALS, METACAUPALS, 

 AND PHALANGES. 



