80 ULNA. 



ULNA. The ulna is a long bone, divisible into a shaft and two 

 extremities. The upper extremity is large, and forms principally the 

 articulation of the elbow ; while the lower extremity is small, and 

 excluded from the wrist-joint by an inter-articular fibro- cartilage. 



The superior esrtremity presents a semilunar concavity of large size, 

 the greater sigmoid notch, for articulation with the humerus ; and upon 

 the outer side a lesser sigmoid notch, which articulates with the head 

 of the radius. Bounding the greater sigmoid notch posteriorly is the 

 olecranon process ; and, overhanging it in front, a pointed eminence 

 with a rough triangular base, the coronoid process. Behind the 

 lesser sigmoid notch, and extending downwards on the side of the 

 olecranon, is a triangular uneven surface, for the anconeus muscle ; 

 and upon the posterior surface of the olecranon a smooth triangular 

 surface, which is subcutaneous. 



The shaft is prismoid in form, and presents three surfaces, 

 anterior, posterior, and internal ; and three borders. The anterior 

 surface is occupied by the flexor profundus digitorum for the upper 

 three-fourths of its extent ; and below by a depression, for the pro- 

 nator quadratus muscle. A little above its middle is the nutritious 

 foramen, which is directed upwards. Upon the posterior surface at 

 the upper part of the bone is the triangular uneven depression for the 

 anconeus muscle, bounded inferiorly by an oblique ridge which runs 

 downwards from the posterior extremity of the lesser sigmoid notch. 

 Below the ridge the surface is marked into several grooves, for the at- 

 tachment of the extensor ossis metacarpi, extensor secundi internodii, 

 and extensor indicis muscle. The internal surface is covered in for the 

 the greater part of its extent by the flexor profundus digitorum. The 

 anterior border is rounded, and gives origin by its lower fourth to the 

 pronator quadratus ; the posterior is more prominent, and affords at- 

 tachment to the flexor carpi ulnaris and extensor carpi ulnaris. At 

 its upper extremity it expands into the triangular subcutaneous sur- 

 face of the olecranon. The external or radial border is sharp and pro- 

 minent, for the attachment of the interosseous membrane. 



The lower extremity terminates in a small rounded head, capitidum 

 ulna, from the side of which projects the styloid process. The latter 

 presents a deep notch at its base for the attachment of the apex of the 

 triangular interarticular cartilage, and by its point gives attachment to 

 the internal lateral ligament. Upon the posterior surface of the head 

 is a groove, for the tendon of the extensor carpi ulnaris ; and upon the 

 side opposite to the styloid process a smooth surface, for articulation 

 with the side of the radius. 



Development. By three centres ; one for the shaft, one for the in- 

 ferior extremity, and one for the olecranon. Ossification commences 

 in the ulna shortly after the humerus and radius ; the two ends of the 

 bone are cartilaginous at birth. The centre for the lower end appears 

 at about the fifth, and that for the olecranon about the seventh year. 

 The bone is completed at about the twentieth year. 



Articulations. With tico bones ; the humerus and radius ; it is se- 



