186 



THE ULNA. 



Ulna. 



The length of this bone is equal to the forearm, of which 

 it is a part. It is thickest above, and gradually diminishes 

 until near its lower end. The body of the bone is nearly 

 Fig. 42.* triangular in form. At the upper extremity 

 ^ of the ulna, on its anterior surface, is a 



semicircular notch. The end of the bone 

 which forms the posterior part of this 

 notch is denominated olecranon. The an- 

 terior part of the notch is formed by a 

 process called coronoid. This notch ap- 

 plies to the pulley-like surface on the internal 

 side of the lower extremity of the os humeri, 

 to form the articulation of the elbow. In 

 the middle of the concave surface is a ridge, 

 in consequence of which, a small rocking 

 motion is performed by the ulna. The 

 external surface of the olecranon is rough, 

 and strongly marked. The extensor mus- 

 cle of the forearm is inserted into the 

 end of it, and below this is a flat surface 

 on which we lean. On the outside of 

 the coronoid process is a semilunated smooth cavity, lined 

 with cartilage; in which, and in a ligament extended 

 from the one to the other end of this cavity, the round 

 head of the radius plays. Immediately below it, a rough 

 hollow gives lodging to the mucilaginous glands. Below the 

 root of the coronoid process, this bone is scabrous and unequal, 

 where the brachialis internus is inserted. On the outside of 



* The two bones of the forearm seen from the front. 1. The shaft of the ulna. 

 2. The greater sigmoid notch. 3. The lesser sigmoid notch, with which the 

 head of the radius is articulated. 4. The olecranon process. 5. The coronoid 

 process. 6. The nutritious foramen. 7. The sharp ridges upon the two bones 

 to which the interosseous membrane is attached. 8. The rounded head at the 

 lower extremity of the ulna. 9. The styloid process. 10. The shaft of the 

 radius. 11. Its head surrounded by the smooth border for articulation with the 

 orbicular ligament. 12. The neck of the radius. 13. Its tuberosity. 14. The 

 oblique line. 15. The lower extremity of the bone. 16. Its styloid process. 



