OF DIDELPHYS VIRGINIANA. 



79 



Fig. 22.— Left Ulna and Radius. 



abruptly externally, instead of gradually subsiding. Certain minor differences aside, the 

 resemblance of this bone to the human humerus is striking. 



Ulna. — (Fig. 22.) Considerably longer than the radius, 

 in consequence of the great backward prolongation of the 

 olecranon and length of the conical lower extremity. The 

 olecranon lies in the axis of the shaft so that the articular cav- 

 ity appears cut out from the upper aspect of the latter. Below, 

 the shaft is nearly straight and cylindrical ; its upper half is 

 much compressed, the posterior border presenting itself as a 

 curved ridge ; and the anterior aspect has also a slight ridge 

 along its middle third, subsiding above into a smooth, oblique, 

 depressed face, along which the radius lies in pronation. This 

 space widens internally, and suddenly curves forward to form 

 the anterior lip of the greater articular cavity ; externally it 

 expands still more, and runs out into a pointed eminence, form- 

 ing the floor of the "lesser sigmoid cavity," in which the head 

 of the radius rests. The two articular cavities are continuous 

 with each other ; the greater lies very oblique to the axis of 

 the shaft. The coronoid process is wider and more prominent 

 than the posterior lip of the facet ; Avhich latter only repre- 

 sents a small part of the whole olecranon. The pointed, conical 

 lower end of the bone is received, and rotates, in a cup-shaped 

 depression, between the cuneiform and pisiform ; the facet for the radial articulation is 

 obscure. (The outline gives the profile of the sigmoid cavity.) 



Radius. — (Fig. 22.) In the supine arm the radius lies very nearly parallel with the 

 ulna ; and the bones are only separated by a narrow interspace. The head of the bone 

 presents the usual shallow circular cup for humeral articulation ; continuous internally with 

 a small convex facet that fits the lesser sigmoid cavity ; rotation of the head, and, conse- 

 quently, pronation of the arm, being perfect. The shaft is straight as far as the tubercle, 



where a gentle outward curvature begins, and thence continues to the lower extremity 



enabling the bone to just clear the ulna during pronation. The stout enlarged lower 

 extremity presents a broad, articular surface, that is prolonged, externally, into a conical 

 process, like that of the ulna. This eminence overlies the scaphoid ; the rest of the head 

 articulates with the semilunar. (The dotted line indicates the radius in pronation.) 



Carpus. — As in man, there are eight carpal bones, four in the proximal and four in the 

 distal row ; there are also two sesamoids on the radial side ; the distal one of which articu- 

 lates so extensively with the trapezium that it might be regarded as a fifth carpal bone 

 proper of the second row. This sesamoid is wedged in between the trapezium and scaph- 

 oid ; the proximal sesamoid is much smaller ; it is merely a flattened oval nodule, liga- 

 mentously connected with the scaphoid and end of the radius. 



The scaphoid has a large, flat, oblique flicet for the radius, a cupped one for the semi- 

 lunar, separated from a smaller, irregular facet for the trapezoid by a sharp process that 

 extends between the semilunar and trapezoid nearly to the magnum. The relation between 

 the scaphoid and trapezium is less intimate than that between it and the other bones just 

 mentioned. The semilunar is a small concavo-convex triangle, with two facets upon its 



