ARTICULATIONS 157 



above its proximal margin, between the two bones of the 

 forearm, whilst the terminal branch of the volar interosseous 

 artery pierces it about two inches above its distal end. This 

 ligament braces the two bones together in such a manner 

 that forces, passing proximally through the radius, are trans- 

 mitted from the radius to the ulna, and it extends the surface 

 of origin for the muscles of the forearm. By its volar 

 surface it gives origin to the flexor digitorum profundus and 

 the flexor pollicis longus muscles, whilst from its dorsal 

 surface springs fibres of the two extensor muscles of the 

 thumb, the abductor pollicis longus, and the extensor indicis 

 proprius. 



Chorda Obliqua (O.T. Oblique Ligament). This is a weak 

 band of fibres which springs from the tuberosity of the ulna, 

 and extends obliquely distally and laterally to find an attach- 

 ment to the radius immediately distal to its tuberosity. It 

 crosses the open space between the bones of the forearm 

 above the proximal border of the interosseous membrane. 

 The chorda obliqua is often absent, and unless the utmost 

 care be taken in removing the adjacent muscles it is apt to 

 be injured. 



Movements at the Radio-ulnar Joints. At these articulations the 

 movements of pronation and supination take place. When the limb is in 

 a condition of complete supination the palm of the hand is directed anteriorly, 

 the thumb laterally, and the two bones of the forearm are parallel, the 

 radius lying along the lateral side of the ulna. In the movement of pronation 

 the radius is thrown across the ulna, so that its distal end comes to lie across 

 the volar surface and on the medial side of the ulna. Further, the hand 

 follows the radius in this movement, and the dorsal aspects of both are 

 directed forwards, and the thumb is turned medially. 



The dissector should analyse, as far as possible in the part upon which 

 he is engaged, the movements at the two radio-ulnar joints which produce 

 these effects. At the same time it should be remembered that results 

 obtained from a limb, in which the dissection has proceeded so far, are apt 

 to be deceptive. 



In the case of the proximal radio-ulnar joint the movement is simple 

 enough. The head of the radius merely rotates within the annular 

 ligament, and accuracy of motion is obtained by the fovea capituli radii 

 resting and moving upon the rounded capitellum of the humerus. But it 

 should be noticed that the head of the radius does not fit accurately upon 

 the capitellum in all positions of the elbow-joint. In extreme extension 

 and extreme flexion of the elbow it is only partially in contact with it. 

 Therefore the semi-flexed condition of the elbow-joint places the radius 

 in the most favourable position for free and precise movement at the 

 proximal radio-ulnar joint. 



At the distal radio-ulnar joint the distal end of the radius revolves 

 around the distal end of the ulna. It carries the hand with it, and describes 

 the arc of a circle, the centre of which corresponds to the attachment of the 



