THE WRIST-JOINT 265 



The synovial membrane, sometimes called the membrana sacciformis, is 

 large and loose in proportion to the size of the joint. It is not only interposed 

 between the radial and ulnar articular surfaces, but lines the terminal articular 

 surface of the ulna and the upper surface of the articular disc. 



The arterial supply is derived from the volar interosseous artery and branches of the 

 volar carpal rete. 



The nerve-supply comes from the volar interosseous of the median, and the deep branch 

 of the radial (posterior interosseous). 



Relations. — -Behind lies the tendon of the extensor digiti quinti proprius and in front the 

 flexor digitorum profundus. 



The movements of the radius. — The upper end of the radius rotates upon an axis drawn 

 through its own head and neck within the collar formed by the radial notch and the annular 

 ligament, while the lower end, retained in position by the articular disc, roUs round the 

 head of the ulna. This rotation is called pronation, when the radius from a position nearly 

 parallel to the ulna turns medialward so as to lie obliquely across it; and supination , when the 

 radius turns back again, so as to uncross and lie nearly parallel with the ulna. In these move- 

 ments the radius carries with it the hand, which rotates on an axis passing along the ulnar side 

 of the hand; thus, the hand when pronated lies with its dorsum upward, as in playing the 

 piano, while when supinated, the palm lies upward- — the attitude of a beggar asking alms. 

 Ward thus expresses the relations of the two extremities of the radius in pronation and supina- 

 tion: 'The head of the radius is so disposed in relation to the sigmoid cavity (ulnar notch) at 

 the lower end that the axis of the former if prolonged falls upon the centre of the circle of which 

 the latter is a segment;' the axis thus passes through the lower end of the ulna at a point at 

 which the articular disc is attached, and if prolonged further, passes through the ring finger. 

 Thus the radius describes, in rotating, a blunt-pointed cone whose apex is the centre of the radial 

 head, and whose base is at the wrist; partial rotation of the bone being unaccompanied by any 

 hinge-like or antero-posterior motion of its head, and pronation and supination occurring with- 

 out disturbance to the parallelism of the bones at the superior radio-ulnar joint. Associated 

 with this rotation in the ordinary way, there is some rotation of the humero-ulnar shaft, which 

 causes lateral shifting of the hand from side to side; thus, with pronation there is some abduc- 

 tion, and with supination some adduction combined, so that the hand can keep on the same 

 superficies in both pronation and supination. The power of supination in man is much greater 

 than pronation, owing to the immense power and leverage obtained by the curve of the radius, 

 and by the attachment of the biceps tendon to the back of the tuberosity. For this reason all 

 our screw-driving and boring tools are made to be used by supination movements. 



In the undissected state, the amount of rotation it is possible to obtain is about 135°, so 

 that neither the palm nor the fore part of the lower end of the radius can be turned completely 

 in opposite directions; yet in the living subject this amount can be greatly increased by rotation 

 of the humero-ulnar shaft at the shoulder-joint. 



Pronation is checked in the living subject by (a) the posterior inferior radio-ulnar ligament, 

 which is strengthened by the connection of the sheath of the extensor tendons with it; (b) the 

 lowermost fibres of the interosseous membrane; (c) the back part of the ulnar collateral and 

 adjacent fibres of the posterior ligament of the wrist, and (d) the meeting of the soft parts on 

 the front of the forearm. 



Supination is checked mainly (a) by the medial ulnar collateral ligaments of the wrist, but 

 partly also by (b) the oblique cord; (c) the anterior inferior radio-ulnar ligament, and (d) the 

 lowest fibres of the interosseous membrane. 



The interosseous membrane serves, from the direction of its fibres downward and medially 

 from the radius to the ulna, to transmit the weight of the body from the ulna to the radius in 

 the extended position of the elbow, as in pushing forward with the arms extended, or in support- 

 ing one's own weight on the hands, the ulna being in intimate contact with the humerus, but 

 not at all with the carpus; while the area of contact of the radius with the humerus is small, 

 and that of the radius with the carpus large. Hence the weight transmitted by the ulna is 

 communicated to the radius by the tightening of the interosseous membrane. Conversely, in 

 falls upon the hand with the arm extended, the interosseous membrane acts as a sling to break 

 the violence of the shock, and prevents the whole force of the impact from expending itself 

 directly upon the capitulum. 



Muscles which act upon the radio-ulnar joints. — Pronators. — Pronator teres, pronator 

 quadratus, flexor carpi radialis, palmaris longus. 



Supinators. — Biceps, supinator, extensor poUicis longus. 



The brachio-radiahs is chiefly a flexor of the elbow-joint, but it takes part in the initiation 

 of the movement of supination when the hand is fully pronated and of pronation when the hand 

 is fully supinated. 



6. THE RADIO-CARPAL OR WRIST-JOINT 

 Class. — Diarthrosis. Subdivision. — Condylarthrosis. 



The wrist-joint is formed by the union of the radius and articular disc above, 

 articulating with the navicular, lunate, and triquetral bones below; the ulna 

 being excluded by the intervention of the articular disc. The radius and disc 

 together present a smooth surface, slightly concave both from before backward, 

 and from side to side, whilst the three bones of the carpus present a smooth, 



