THE RADIO-CARPAL 233 



is called 'pronation, when the radius from a i)Osition nearly parallel to the ulna turns 

 inwards so as to lie oliliquely across it; and sirplriation, when the radius turns V)ack 

 again from within outwards, so as to uncross and lie nearly parallel with the ulna. 

 In these movements, the radius carries with it the hand, Avhich rotates on an axis 

 passing a little to the inner side of the middle line; thus, the hand when })ronated, 

 lies with its dorsum upwards, as in playing the piano, while Avhen sui)inated, the 

 ])alm lies upwards — the attitude of a beggar asking alms. "W'ard thus expresses 

 the relations of the two extremities of the radius in pronation and supination: ' The 

 head of the radius is so disposed in relation to the sigmoid cavity at the lower end 

 tliat the axis of the former if prolonged falls upon the centre of the circle of which 

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

 a point at which the triangular fibro-cartilage is attached, and if prolonged further, 

 l)asses througli the ring linger. Thus the radius describes, in rotating, a Idunt- 

 })ointed cone whose apex is the centre of the radial head, and whose base is at the 

 wrist; partial rotation of the Ijone being unaccompanied by any hinge-like or 

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

 out disturliance 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 abduction, 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 ]iower and leverage obtained l)y the curve of the radius, 

 and by the attaclnnent of the l)iceps tendon to the back of the tubercle. For this 

 reason all our screw-driving and l)oring tools are made to he used by supination 

 movements. 



In the undissected state, the amount of rotation it is possible to ol)tain is aljout 

 135°, so that neither the palm nor the fore part of the lower end of the radius can 

 ])e 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, Avhich is strengthened by the connection of the sheath of the extensor 

 tendons with it; (b) the lowermost fibres of the interosseous meml)rane; (c) the 

 l)ack part of the internal 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 internal lateral ligament of the wrist. 

 l)ut partly also by (b) the ol)lique radio-ulnar ligament; (c) the anterior inferior 

 radio-ulnar ligament, and (d) the lowest fibres of the interosseous membrane. 



The interosseous membrane ser\'es, from the direction of its filires downwards 

 and inwards 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 forwards 

 with the arms extended, or in supporting one's own weight on the hands, the ulna 

 being in close contact with the humerus, but scarcely at all with the carpus; Avhile 

 the radius is scarcely in contact with the humerus, but in close contact with the 

 carpus. Hence the weight transmitted l)y the ulna is communicated to the radius 

 l)y the tightening of the interosseous membrane. Conversely, it falls upon the hand 

 with the arm extended, the interosseous membrane acting as a sling to break the 

 violence of the shock, and preventing the whole force of the impact from expend- 

 ing itself directly upon the capitellum. 



6. THE RADIO-CARPAL ARTICULATIOX 



Class. — Dimihrosis. Subdivision. — Condi/larthrosis. 



The wrist-joint is formed by the union of the radius and triangular fil)ro-car- 

 tilage above, articulating with the scaphoid, semilunar, and cuneiform l)ones lielow; 

 the ulna being excluded by the intervention of the triangular fibro-cartilage. The 

 radius and triangular cartilage together i)resent a smooth surface, slightly concave 

 both from before backwards, and from side to side; whilst the three l)ones of the 



