RADIUS. 107 



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 occu- 

 pied by the flexor profundis digitorum for the upper three-fourths of its 

 extent ; and below by a depression, for the pronator quadratus muscle. 

 A little above its middle is the nutritious foramen, which is directed up- 

 wards. 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 attachment of the extensor ossis metacarpi, exten- 

 sor secundi internodii, and extensor indicis muscle. The internal surface 

 is covered in for the greater part of its extent by the flexor profundis digi- 

 torum. The anterior border is rounded, and gives origin by its lower 

 fourth to the pronator quadratus ; the posterior is more prominent, and 

 affords attachment to the flexor carpi ulnaris and extensor carpi ulnaris. 

 At its upper extremity it expands into the triangular subcutaneous surface 

 of the olecranon. The external or radial border is sharp and prominent, 

 for the attachment of the interosseous membrane. 



The lower extremity terminates in a small rounded head, capitulum 

 ulnce, from the side of which projects the styloid process. The latter pre- 

 sents a deep notch at its base for the attachment of the apex of the trian- 

 gular 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 inferior 

 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 two bones ; the humerus and radius ; it is sepa- 

 rated from the cuneiform bone of the carpus by the triangular interarticular 

 cartilage. 



Attachment of Muscles. To twelve ; by the olecranon, to the triceps 

 extensor cubiti, one head of the flexor carpi ulnaris, and the anconeus ; 

 by the coronoid process, to the brachialis anticus, pronator radii teres, 

 flexor sublimis digitorum, and flexor profundus digitorum ; by the shaft, 

 to the flexor profundus digitorum, flexor carpi ulnaris, pronator quadratus, 

 anconeus, extensor carpi ulnaris, extensor ossis metacarpi pollicis, extensor 

 secundi internodii pollicis, and extensor indicis. 



RADIUS. The radius is the rotatory bone of the fore-arm ; it is divisible 

 into a shaft and two extremities : unlike the ulna, its upper extremity is 

 small, and merely accessory to the formation of the elbow-joint; while the 

 lower extremity is large, and forms almost solely the joint of the wrist. 



The superior extremity, presents a rounded head, depressed upon iti 



