RADIO- ULNAR ARTICULA TIONS. 



261 



placed transversely beneath the head of the ulna, binding the lower end of this 

 bone and the radius firmly together. Its periphery is thicker than its centre, which 

 is thin and occasionally perforated. It is attached by its apex to a depression 

 which separates the styloid process of the ulna from the head of that bone ; and 

 by its base, which is thin, to the prominent edge of the radius, which separates the 

 sigmoid cavity from the carpal articulating surface. Its margins are united to the 

 ligaments of the wrist-joint. Its upper surface, smooth and concave, articulates 

 with the head of the ulna, forming an arthrodial joint ; its under surface, also 

 concave and smooth, forms part of the wrist-joint and articulates with the cuneiform 

 and inner part of the semilunar bone. Both surfaces are lined by a synovial 

 membrane the upper surface, by one peculiar to the radio-ulnar articulation ; the 

 under surface, by the synovial membrane of the wrist. 



Inferior radio-ulnar 



articulation. 



Carpal articulations. 



Carpo-metacarpal 



articulations. 



FIG. 175. Ligaments of wrist and hand. Posterior view. 



The Synovial Membrane (Fig. 177) of this articulation has been called, from 

 its extreme looseness, the membrana sacciformis ; it extends horizontally inward 

 between the head of the ulna and the interarticular fibro-cartilage, and upward 

 between the radius and the ulna, forming here a very loose cul-de-sac. The quan- 

 tity of synovia which it contains is usually considerable. 



Actions. The movement in the inferior radio-ulnar articulation is just the 

 reverse of that in the superior radio-ulnar joint. It consists of a movement of 

 rotation of the lower end of the radius round an axis which corresponds to the 

 centre of the head of the ulna. When the radius rotates forward, pronation of the 

 forearm and hand is the result ; and when backward, supination. It will thus be 

 seen that in pronation and supination of the forearm and hand the radius describes 

 a segment of a cone, the axis of which extends from the centre of the head of 

 the radius to the middle of the head of the ulna. In this movement, however, 

 the ulna is not quite stationary, but rotates a little in the opposite direction. 

 So that it also describes the segment of a cone, though of smaller size than that 

 described by the radius. The movement Avhich causes this alteration in the posi- 

 tion of the head of the ulna takes place principally at the shoulder-joint by a rota- 

 tion of the humerus, but possibly also to a slight extent at the elbow-joint. 1 



Surface Form. The position of the inferior radio-ulnar joint may be ascertained by 

 feeling for a slight groove at the back of the wrist, between the prominent head of the 

 ulna and the lower end of the radius, when the forearm is in a state of almost complete prona- 

 tion. 



1 See Journ. of Anal, and Phys., vol. xix., parts ii., iii., and iv. 



