OSTEOLOGY AND ARTHROLOGY 55 



where the inner Hp of the trochlea is lower than the 

 condyle. 



In the dog, the olecranon fossa communicates with the 

 coronoid by an opening. 



In the cat, there is a supra-epitrochlear canal (see Fig. 19), 

 but no olecranon perforation. 



The bones of the forearm articulate at their extremities. 

 The body of the radius is united to the body of the ulna by 

 a short, thick, interosseous hgament ; the fibres of this 

 ligament, though short, do not prevent the production of 

 some movements at the articulations of the bones. 



The radius so crosses the ulna that above, it is in 

 front and external to the latter, while below, it is internal. 

 This bone is flattened from front to back, and slightly 

 convex anteriorly. Its superior extremity is formed, 

 externally, of a portion which represents the head of the 

 radius in man ; internally, by another portion which 

 represents half of the coronoid process of the ulna, which, 

 in the human being, belongs exclusively to the latter (see 

 p. 39, the encroachment of the radius on the ulna). This 

 extremity is surrounded with a vertical articular surface 

 which is placed in contact with a small cavity which is 

 hollowed out on the ulna (the lesser sigmoid cavity) ; and 

 presents at its superior aspect a surface which articulates 

 with the inferior extremity of the humerus. The shaft of 

 the bone has on its internal border rugosities analogous to 

 the imprint of the pronator radii teres of the human skeleton ; 

 these rugosities, indeed, give insertion to a muscle of the 

 same function, and bearing the same name. The inferior 

 extremity, broader than the superior, is hollowed on its 

 external aspect by a small cavity which receives the in- 

 ferior extremity of the ulna ; its inferior surface (concave) 

 articulates with the carpus ; its anterior surface (the homo- 

 logue of the posterior surface of the corresponding extremity 

 of the human radius) presents grooves which serve for 

 the passage of the tendons of the muscles which pass 

 from the forearm to the back of the hand. (For the 

 names of the muscles whose tendons pass in these grooves, 

 see Fig. 29.) 



