570 MAMMALIAN ANATOMY 



The lesser sigmoid cavity is concave transversely, and is shallower 

 than the greater sigmoid cavity. It faces outward. Its long diameter 

 is almost transverse to the long axis of the ulna, passing backward and 

 somewhat proximally, and is three times greater than the proximo- 

 distal diameter. The rounded outer end of the cavity is supported by 

 a prominent lip for ligamentous attachment, the posterior surface of 

 which overhangs the groove for the extensor indicis muscle. The 

 inner anterior end is pointed and forms the outer surface of the 

 coronoid process. The upper and lower margins of the cavity are 

 parallel ; the upper slightly arcuate, the lower faintly emarginate at 

 the middle. The lesser sigmoid cavity receives the anterior side of 

 the head of the radius and permits of some rotatory motion. 



The coronoid process is a small pyramid, the apex of which points 

 outward, downward, and forward when the bone is held vertically. 

 Its sharp anterior border separates the outer surface belonging to the 

 lesser sigmoid cavity from a smooth, triangular, gently convex ante- 

 rior surface, which faces forward, inward, and upward, is covered 

 with articular cartilage, and enters into the elbow-joint. The tri- 

 angular posterior surface of the coronoid process is rough for the 

 attachment of the capsular ligament of the articulation, and is marked 

 at its base by a transverse groove for the internal lateral ligament 

 from the humerus. 



The Shaft of the ulna comprises almost the entire bone. In the 

 proximal third it is compressed from side to side, but is deep from 

 before backward ; in the lower two-thirds it is triangular on cross 

 section. Its antero-posterior diameter becomes less toward the inferior 

 extremity. From above downward the shaft is bowed slightly forward; 

 it has a sinuous curve from side to side, at first bending from the supe- 

 rior extremity outward at the middle point, then it gradually turns and 

 at the lower end is again directed slightly outward. Three surfaces, 

 the anterior, the internal, and the posterior, and three borders, the 

 external, the anterior, and the posterior, may be distinguished on the 

 shaft of the ulna. 



The anterior border (Fig. 442) begins above on the inner side of 

 the greater sigmoid cavity, at the roughened area of attachment of the 

 internal lateral ligament ; it follows the curves on the bone and ends on 

 the inner side of the lower extremity. The border is not distinct in 

 its proximal two-thirds, where it rather represents the maximum con- 



