328 THE AETICULATIONS OR JOINTS. 



wards between the bones to reach the dorsal aspect of the forearm. The fibres 

 which compose the interosseous membrane run for the most part dis tally and 

 medially from the radius to the ulna, although on its dorsal aspect several bands 

 may be observed stretching in an opposite direction. The interosseous membrane 

 augments the surface available for the origin of the muscles of the forearm ; it 

 braces the radius and ulna together ; and when shocks are communicated from 

 the hand to the radius, owing to the direction of its fibres, the interosseous 

 membrane transmits them, to a large extent, to the ulna. 



Movements of the Radius on the Ulna. The axis around which the radius moves is a 

 longitudinal one, having one end passing through the centre of the head of the radius and 

 the other through the styloid process of the ulna and the line of the ring-finger. In this axis 

 the head of the radius is so secured that it can only rotate upon the radial notch of the 

 ulna within the annular ligament of the radius, and consequently the radial head remains upon 

 the same plane as the ulna ; but the distal end of the radius being merely restrained by 

 the articular disc, is able to describe nearly a half-circle, of which the apex of this ligament is 

 the centre. In this movement the radius carries the hand from a position in which the palm is 

 directed forwards, and in which the radius and ulna lie parallel to each other (supination), to 

 one in which the palm is directed backwards, and the radius lies diagonally across the front of 

 the ulna (pronation). 



The ulna is unable to rotate upon a long axis, but while the radius is travelling through the 

 arc of a circle from lateral to medial side in front of the ulna, it will usually be seen that the 

 ulna appears to move through the arc of a smaller circle in the reverse direction, viz., from 

 medial to lateral side. If the humerus is prevented from moving at the shoulder-joint, a very 

 large proportion, if not the entire amount, of this apparent movement of the ulna will disappear. 

 At the same time some observers maintain that it really occurs at the elbow-joint, associated with 

 lateral movement during slight degrees of flexion and extension at that joint. 



ARTICULATIO RADIOCARPEA. 



The radio-carpal joint is a bi-axial diarthrosis, frequently called a condyloid 

 joint. 



The articular elements which enter into its formation are : on its proximal side, 

 the carpal surface of the distal end of the radius, together with the distal surface 

 of the discus articularis ; on its distal side, the proximal articular surfaces of the 

 navicular, lunate, and triquetral bones, and the interosseous ligaments between 

 them. The articular surface of the radius is concave both in its antero-posterior 

 and transverse diameters, in order to adapt itself to the opposing surfaces of the 

 navicular and lunate, which are convex in the two axes named. In the ordinary 

 straight position of the hand the articular disc is in contact with the lunate bone, 

 and the proximal articular surface of the triquetral bone is in contact with the 

 capsule of the joint. When, however, the hand is bent towards the ulna the 

 triquetral bone is carried laterally as well as' the lunate and navicular and the 

 articular disc comes into contact with the triquetral. The articular surface of the 

 radius is subdivided by an antero-posterior, slightly elevated ridge, into a lateral 

 triangular facet which usually articulates with the navicular, and a medial 

 quadrilateral facet for articulation with a portion of the lunate bone. 



In the intervals between the navicular, lunate, and triquetral bones, the con- 

 tinuity of the distal articular surface is maintained by the presence of interosseous 

 ligaments which are situated upon the same level as the articular cartilage. 



Capsula Articularis. An articular capsule completely surrounds the joint. It 

 is somewhat loosely arranged, and its fibrous stratum permits of subdivision into 

 the following four portions : 



Lig. Radiocarpeum Laterale. The lateral radio-carpal ligament (0. T. external 

 lateral) (Fig. 310) is a well-defined band which is attached by one end to the tip 

 of the styloid process of the radius, and by the other to a rough area at the base 

 of the tuberosity of the navicular bone, i.e. lateral to its radial articular surface. 



Lig. Ulnocarpeum Mediate. The medial ulno-carpal ligament (O.T. internal 

 lateral) (Fig. 310) is also a distinct rounded structure, having one end attached to 

 the tip of the styloid process of the ulna, and the other to the rough non-articular 

 border of the triquetral bone, some of its fibres being prolonged to the pisiform bone. 



Lig. Radiocarpeum Volare. The volar radio-carpal ligament (O.T. anterior 

 ligament) (Fig. 310) is attached proximally to the volar margin of the distal end 



