THE HUMERUS 85 



articular surface, separated by a rounded ridge; the inner 

 condyle is five-sixths articular. The prominent tuberosities 

 situated on either condyle are the epicondyles, developed from 

 separate centres. The internal epicondyle is the more promi- 

 nent one, is inclined backward, and forms posteriorly a shallow 

 groove for the ulnar nerve. It gives attachment to the pro- 

 nator radii tercs and the common tendon of the superficial 

 pronatoflexor muscles of the forearm. 



The external condyle presents (1) the epicondyle, which 

 gives origin to some of the supinatoextensor muscles of the 

 forearm; (2) below and internal to this on the condyle a small 

 impression for the anconeus; and (3) a pit for the external 

 lateral ligament. 



The inferior articular surface is divided into two parts: 

 The external part, rounded and directed forward, is the capi- 

 tellum for articulation with the radius; it does not extend at 

 all on the posterior surface. Internal to it is a groove for the 

 inner margin of the head of the radius. The internal portion, 

 or trochlea, articulates with the ulna, and extends upon the 

 anterior and the posterior surface of the bone; the external 

 border is rounded and corresponds to the interval between 

 the radius and ulna. The internal border is thick and promi- 

 nent. Anteriorly these margins are inclined down and inward, 

 posteriorly up and outward, so that the groove is obliquely 

 inclined from without inward, and if continued would form 

 the thread of a screw. The external part of the trochlea is 

 the segment of a sphere, the internal part the segment of a 

 truncated cone with base internal; at the junction of the cone 

 and sphere is the groove. 



Above the trochlea posteriorly is the olecranon fossa, above 

 it anteriorly the coronoid fossa; the thin plate between them 

 may be perforated by the supratrochlear foramen. This occurs 

 more often in the lower races of man. Above the capitellum 

 is the radial fossa for the head of the radius in flexion. 



The average length of the adult male humerus is 13 inches; 

 female, 12 inches. It is nearly one-fifth the height of the 

 individual. The right humerus with the radius is usually 

 % to f inch longer than the left; but there is no difference at 

 birth. 



The shaft of the humerus is twisted through about 135 

 degrees. The twist is seen at the spiral groove, "groove of 



