68 THE SUPERIOR EXTREMITY 



On the lateral aspect of the wrist, a triangular depression is 

 visible when the thumb is extended. It is bounded antero- 

 laterally by the tendons of the Abductor Pollicis Longus and 

 the Extensor Pollicis Brevis and dorso-medially by the 

 Extensor Pollicis Longus. The floor of this fossa is crossed 

 obliquely by the second part of the radial artery (p. 92). 



On the back of the wrist the tendons of the Extensor Carpi 

 Radialis Longus and Brevis can be seen when the fist is tightly 

 clenched, and they can be traced to their insertions into the 

 bases of the second and third metacarpal bones respectively. 



Muscles of the Forearm and their Relations to 

 Fractures. The superficial group of flexors and pronators 

 arises mainly from the medial epicondyle of the humerus. They 

 pass with varying degrees of obliquity down the forearm and 

 are all, with the exception of the flexor carpi ulnaris, supplied 

 by branches given off from the trunk of the Median Nerve 

 (C. 6), just distal to the elbow-joint. 



The Pronator Teres, which has in addition a deep head of 

 origin from the coronoid process, is the most lateral and most 

 obliquely directed of this group. It is inserted into the middle 

 of the lateral surface of the radius and is thus a powerful pronator 

 of the forearm, while it is also a flexor of the elbow. In tennis- 

 players this muscle is frequently overstretched or strained. 



Fractures of the Bones of the Forearm. Fractures 

 from indirect violence may affect both bones or the radius alone. 

 They occur from falls on the hand, and only a small part of the 

 shock is directly transmitted to the ulna. The greater part is 

 communicated to the expanded distal end of the radius and 

 passes upwards to the humerus. Only a small degree of the 

 shock is transmitted from the radius to the ulna by the 

 interosseous membrane, the fibres of which are mainly directed 

 distally and medially. Fracture of the shaft of the ulna alone 

 is always due to direct violence and tends to be compound, as 

 the dorsal border of the bone is subcutaneous in its whole 

 extent. 



In Fractures between the Radial (Bicipital) Tuberosity 

 and the Insertion of the Pronator Teres, the small proximal 

 fragment is flexed and supinated by the biceps. The distal 

 fragment is pronated by the pronator muscles and its proximal 

 end is tilted towards the ulna by the brachio-radialis and the 

 pronator teres. The proximal fragment is difficult to control 

 owing to its small size, and it is necessary to bring the distal 



