142 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.YI. 



Tbe trocMea surface does not extend inwards very far; i. e., does not 

 pass over the end of the bone. Above it and towards the proximal end 

 api)ears a distinct and prominent " external condyle". 



Anconad this extremity of the humerus presents for examination the 

 upturned internal condjde and a longitudinal tendinal groove, situated 

 ojDposite the radial convexity, with intervening indentations. This ar- 

 rangement lends to this aspect of the bone rather an uneven and tuber- 

 ous look. The nutrient foramen, almost too minute to be observed by 

 the naked eye, is found at the middle and inner aspect of the shaft. 



The radius is a long, delicate bone, with a bent and compressed shaft. 

 A moderately well-expanded and circular " head " presents the usual 

 concavity for the oblique tubercle of the humeral trochlea, while below 

 is a feebly marked " ulnar facet " and bicipital tuberosity ; beyond this, 

 again, the shaft develops a sharp, protruding edge, that extends nearly 

 to mid-shaft and into the interosseous space. 



The distal extremity of this bone is spread transversely and curved 

 downwards. It articulates with the upper surface and distal end of the 

 ulna, and is lined above by very minute tendinal grooves. The outer 

 border of this extremity presents a transverse lamina of bone that seems 

 to be superadded to the truly dilated end. 



In articulation, the radius at first curves away from its companion, 

 the ulna, to ai)proach it again towards the carpal end, for about the outer 

 third of the shaft, to remain with it until both arrive at the wrist. The 

 distal border of the radius is transversely convex for an articular facette 

 on the scapho-lunar. The ulna is the main bony support of the forearm, 

 and, indeed, its shaft is nearly equal in size and strength to that of the 

 humerus itself, having the apj)earance of being the true continuation of 

 the pectoral limb, so diminutive and slender is the accompanying radius. 



Its proximal extremity is the larger, and is gently curved anconad, 

 to meet the corresponding flexure of the brachium to form the elbow- 

 joint, the articular surface engaged being quite extensive and vertically 

 expanded. The lower, circular, and markedly concave trochlea is the 

 greater sigmoid cavity, and is intended for the ulnar tubercle of the 

 humerus. Its proximal margin is so produced as to form a strongly 

 defined "olecranon i^rocess", the lower lip of the cavity being thehomo- 

 logue of the " coronoid process", and is so feebly developed as to scarcely 

 deserve the distinction. In close i^roximity to the greater sigmoid cav- 

 ity, above, there is another articular surface, quadrilateral in outline, 

 decidedly concave from above downwards, much more shallow in the 

 opposite direction, for the oblique tubercle. Immediately beyond its 

 distal margin is a weak and shallow facette for the side of the head of 

 the radius, so that the oblique tubercle articulates in a cavity furnished 

 by the cupped-head of the radius and the larger quadrilateral trochlea 

 of the cubitus, the two being almost continuous. 



The outer aspect of this extremity presents simply certain feeble ele- 

 vations and depressions for muscles and the ligaments surrounding the 

 joint. 



