3o8 HUMAN ANATOMY. 



in the groove between the internal condyle and olecranon. (3) The close relation 

 of the posterior interosseous nerve to the head of the radius. (4) The post-operative 

 value (^in extending the forearm) of the outer aponeurotic expansion of the triceps 

 and of the anconeus muscle. These should be carefully protected from injury. 



Landmarks. — The following points may be mentioned in addition to those 

 which may be found under the Humerus, Radius, and Ulna : 



A line from one condyle to the other will be at right angles with the humeral 

 axis, but will be oblique in relation to the axis of the forearm. 



The line of the radio-humeral articulation is horizontal. The line of the humero- 

 ulnar articulation is oblique downward and inward ; the tip of the internal condyle 

 is therefore from a quarter to a half inch farther above the articular line than is the 

 tip of the external condyle. The internal condyle points backward rather than 

 inward. 



The length of the articulation line is about two-thirds of the length of a line 

 joining the tips of the condyles. In semiflexion the external condyle is easily seen ; 

 in acute flexion it disappears, and the rounded capitellum of the humerus, with the 

 outer edge of the triceps stretched over it, can be seen and felt. 



The Inferior Radio-Ulnar Joint. — This articulation has been dislocated in 

 a few instances, in most of- which, the cause having been extreme pronation of the 

 wrist, the lower end of the ulna was carried backward, projecting on the back of the 

 wrist and pointing outward, — i.e., towards the middle finger. The backward dis- 

 placement probably involves the tearing of the triangular fibro-cartilage and a rup- 

 ture of the posterior radio-ulnar ligament. The deviation of the ulna to the radial 

 side may be due to the action of the pronator quadratus. The shallowness of the 

 sigmoid cavity on the radius favors recurrence after reduction. But little is known 

 of this injury. 



