CHAP. IV.] 



THE SKELETON. 



27 



the shoulder blade is a shallow socket, the glenoid cavity, which 

 receives the head of the humerus. 



The humerus is the longest and 

 largest bone of the upper limb. 

 The upper extremity of the bone 

 consists of a rounded head joined 

 to the shaft by a constricted neck, 

 and of two eminences called the 

 greater and lesser tuberosities. The 

 head articulates with the glenoid 

 cavity of the scapula. The con- 

 stricted neck above the tuberosities 

 is called the anatomical neck, and 

 that below the tuberosities the sur- 

 gical neck, from its being often the 

 seat of fracture. The lower ex- 

 tremity of the bone is flattened 

 from before backwards into a broad 

 articular surface, which is divided 

 by a slight ridge into two parts, by 

 means of which it articulates with 

 ulna and radius. 



The ulna (elbow bone) is placed 

 at the inner side of the forearm, 

 parallel with the radius. Its upper 

 extremity presents for examination 

 two large curved processes and two 

 concave cavities; the larger process 

 forms the head of the elbow, and 

 is called the olecranon process. The 

 lower extremity of the ulna is of 

 small size, and is excluded from the 

 wrist by a piece of fibro-cartilage. 



The radius is situated on the outer 

 side of the forearm. The upper 

 end is small and rounded with a 

 shallow depression on its upper sur- f. "" 

 face for articulation with the hume- 

 rus, and a prominent ridge about it, like the head of a nail 

 by means of which it rotates within the lesser sigmoid cav- 



FIG. 18. THE HUMERUS. a, 

 rounded head ; gt, greater tuber- 

 osity; It, lesser tuberosity; 6, 



