152 COUES ON THE OSTEOLOGY 



along its inner aspect. Its external border is marked by oblique ridges corresponding 

 to tbe insertions of the secondary quills. The olecranon is a somewhat compressed process 

 at the posterior aspect of the head of the bone, but does not extend above the margins of 

 the articular cavity. This latter is placed upon the inner and anterior aspect of the head 

 of the bone, and has been already described. Just external to, and below it, is the radio- 

 ulnar articulation. It is merely a slight subtriangular depression, against which the head 

 of the radius is apposed, and has leading from its superior margin the continuation of the 

 facet on the head of the radius, for articulation with the external oblique tuberosity of the 

 humerus. Both lateral surfaces of the head of the ulna are rough, for the attachment of 

 muscles. 



The distal extremity of the ulna is fully as large as the proximal, but of quite a different 

 shape. It is greatly flattened horizontally, the anterior margin projecting forwards as a flat 

 ridge, over the superior surface of which passes the radius. The articulation of the radius 

 with the ulna is however merely by a small facet on the middle of this superior aspect of 

 the head of the bone. The greater part of the remaining surface of the head of the bone 

 forms a smooth, somewhat semilunar depression, by which it is articulated, partly with the 

 posterior half of the head of the metacarpal bone, and partly with the small carpal bone, as 

 is more particularly described further on. 



Radius. The radius is exactly of the same length as the ulna, but much smaller than it 

 throughout its whole extent. It is quite straight except just at its distal extremity, where 

 it is somewhat decurved to overlap the extremity of the ulna. Towards its lower extremity 

 it increases in size, though not to any great degree, and becomes horizontally flattened. 

 Its rounded head, considerably larger than the shaft below it, bears upon its inferior 

 border the small ulnar articulating facet, and upon its face the much larger and somewhat 

 oval depression for articulation with the humerus. Its distal extremity is marked supe- 

 riorly with a very deep groove, which continues for some distance up the shaft for the 

 lodgment of the tendon of the extensor metacarpi radialis. Just internal to it is another 

 groove, but much shorter and less conspicuous. 



The radius articulates below with the ulna, as already described ; but its principal articu- 

 lation is with the large bone of the carpus. Nearly the whole of its extremity is occupied 

 by an oval facet, long in its horizontal diameter and transversely very convex, which is 

 received into the deep concavity on the radial aspect of the carpal bone. This radio-carpal 

 articulation presents just the opposite feature from that of man, a convexity of the end of 

 the radius being received into a concavity in the carpal bone, instead of the reverse. 



Bones of the Carpus. Of the two carpal bones, that corresponding to the scaphoid, semilu- 

 nar, and cuneiform, is the largest, and articulates chiefly with the radius. It is wedged in at 

 the outer angle of the joint between the metacarpal bone and the radius, extending far 

 enough towards the ulnar side of the joint to articulate with the ulna by a small facet. 

 It is of a very irregular shape, marked with three facets, for articulation respectively 

 with the radius, ulna, and metacarpus, and with two deep grooves for the lodgement of 

 tendons. 



The deep radial facet has been already noticed. Just on the opposite side from it, 

 the face of the bone presents the smooth depression for the reception of the head of the 

 metacarpus ; this facet is divided into two parts \>y a vertical median ridge, corresponding 

 to the vertical depression in the head of the metacarpus. The very small facet upon the 

 interno-posterior aspect of the bone articulates with the radial edge of the large terminal 

 facet of the ulna. 



