THE HUMERUS. 



147 



The posterior surface (Fig. 98) appears some- 

 what twisted, so that its upper part is directed a 

 little inward, its lower part backward and a little 

 outward. Nearly the whole of this surface is 

 covered by the external and internal heads of 

 the Triceps, the former of which is attached to 

 its upper and outer part, the latter to its inner 

 and back part, the two being separated by the 

 niusculo-spiral groove. 



The Lower Extremity is flattened from before 

 backward, and curved slightly forward; it ter- 

 minates below in a broad, articular surface which 

 is divided into two parts by a slight ridge. Pro- 

 jecting on either side are the external and inter- 

 nal condyles. The articular surface extends a 

 little lower than the condyles, and is curved 

 slightly forward, so as to occupy the more ante- 

 rior part of the bone; its greatest breadth is 

 in the transverse diameter, and it is obliquely 

 directed, so that its inner extremity occupies a 

 lower level than the outer. The outer portion of 

 the articular surface presents a smooth, rounded 

 eminence, which has received the name of the 

 capitellum, or radial head of the humerus; it 

 articulates with the cup-shaped depression on 

 the head of the radius, and is limited to the 

 front and lower part of the bone, not extending 

 as far back as the other portion of the articular 

 surface. On t'he inner side of this eminence is 

 a shallow groove, in which is received the inner 

 margin of the head of the radius. Above the 

 front part of the capitellum is a slight depression, 

 the radial fossa, which receives the anterior 

 border of the head of the radius when the fore- 

 arm is flexed. The inner portion of the articular 

 surface, the trochlea, presents a deep depression 

 between two "well-marked borders. This surface 

 is convex from before backward, concave from 

 side to side, and occupies the anterior, lower, 

 and posterior parts of the bone. The external 

 border, less prominent than the internal, corre- 

 sponds to the interval between the radius and the 

 ulna. The internal border is thicker, more 

 prominent, arid consequently of greater length, 

 than the external. The grooved portion of the 

 articular surface fits accurately within the 

 greater sigmoid cavity of the ulna : it is broader 

 and deeper on the posterior than on the anterior 

 aspect of the bone, and is inclined obliquely from 

 behind forward and from without inward. Above 

 the front part of the trochlear surface is seen a 

 smaller depression, the coronoid fossa, which 

 receives the coronoid process of the ulna during 

 flexion of the forearm. Above the back part of 

 the trochlear surface is a deep, triangular depres- 

 sion, the olecranon fossa, in which is received the 

 summit of the olecranon process in extension of 



\Troch( 



. Left humerus. Posterior surface. 



