64 THE UPPER EXTREMITY 



on the dorsal aspect of the limb. It becomes apparent to 

 the eye as a prominence when the elbow is semi-flexed. The 

 olecranon produces a marked projection on the dorsum of the 

 elbow between the two epicondyles. It is placed slightly 

 nearer to the medial than to the lateral epicondyle. The 

 loose skin which covers the olecranon moves freely over its 

 subcutaneous surface, owing to the interposition of a bursa 

 mucosa. The different positions which are assumed by the 

 olecranon, in relation to the epicondyles of the humerus in 

 the movements of the forearm at the elbow-joint, must be 

 carefully examined. This can best be done by placing the 

 thumb on one epicondyle, the middle finger on the other, 

 and the forefinger on the olecranon. The limb should then 

 be alternately flexed and extended, so as to make clear the 

 extent of the excursion performed by the olecranon. In 

 full extension at the elbow-joint the three prominences 

 lie in the same horizontal line ; when the forearm is bent at 

 a right angle the three bony points are placed at the angles of 

 an equilateral triangle, the apex of which points distally. 



When the forearm is extended a marked depression on the 

 dorsal aspect of the elbow indicates the position of the articula- 

 tion between the radius and the humerus. Immediately distal 

 to this the head of the radius lies close to the surface, and can 

 readily be felt, especially when it is made to roll under the 

 finger by inducing alternately the movements of pronation 

 and supination. The head of the radius is placed about an 

 inch distal to the lateral epicondyle. 



As the skin of the forearm must be reflected in the pur- 

 suit of the cutaneous nerves, it is well, at this stage, to study 

 also the external anatomy of this segment of the limb. In its 

 proximal half the radius is deeply imbedded in muscles, but in 

 its distal half it can be felt, and its styloid process on the 

 lateral side of the carpus can be readily distinguished. On the 

 dorsal aspect of the distal end of the radius immediately 

 above the radiocarpal joint, and nearer the lateral than the 

 medial border of the limb, a prominent bony tubercle may be 

 felt. This is the high ridge which forms the lateral wall of 

 the sharply cut groove on the dorsal aspect of the radius in 

 which the tendon of the extensor pollicis longus muscle plays. 



The sinuous dorsal border of the ulna is subcutaneous, 

 and may be followed by the finger throughout its entire 

 length ; as the elbow is approached it leads directly on 



