12 HANDBOOK OF ANATOMY 



On the outer side of the upper end of the shaft is another articular 

 surface, the radial notch, for articulation with the head of the 

 radius. The posterior surface of the olecranon is smooth and 

 subcutaneous. Just below the coronoid process is a rough 

 tuberosity for the insertion of brachialis anticus. The shaft 

 is triangular for about two-thirds of its length, then tapers 

 gradually, and becomes smooth and rounded. It has three sur- 

 faces anterior, inner, and outer and three borders posterior, 

 inner, and interosseous. The posterior border is subcutaneous 

 throughout its .length. The lower end of the bone is much 

 smaller than the upper, and nearly circular. On its inner surface 

 it has a projection, the styloid process, pointing downwards, 

 and on its outer surface an articular facet for the lower end of 

 the radius. The inferior surface is smooth for articulation 

 with the triangular nbro-cartilage of the wrist- joint. 



The ulna articulates above with the trochlear surface of the 

 humerus by means of the semilunar notch, and below with the 

 articular disc of the wrist-joint ; on its outer surface it articulates 

 with the radius at either extremity. 



Ossification. The centre for the shaft appears before birth, 

 and secondary centres for the olecranon process and the lower 

 end of the shaft appear later to form epiphyses which unite with 

 the shaft in adult life. 



The Radius, the bone on the outer side of the forearm, is a 

 long bone with a shaft and two extremities it differs from the 

 ulna in having a small rounded head and a shaft which gradually 

 widens out so that the lower extremity is much larger and 

 triangular in shape. The head is circular, with a cup-shaped 

 depression on its superior surface; it has a narrow articular 

 surface all round for articulation with the radial notch of the 

 ulna. Immediately below the head it is somewhat constricted to 

 form a neck, and then widens out again into the shaft. The 

 shaft is triangular in section, having three surfaces anterior, 

 outer, and posterior and three borders, the interosseous one 

 being the only well-defined one, as the surface is rounded and 

 confluent with the other two. At the upper and inner side of the 

 anterior surface is the bicipital tuberosity for the insertion of 

 the biceps, and from the lower jedge of that the oblique line 



