274 THE ARTICULATIONS OR JOINTS. 
formation of the elbow-joint, consequently the joint cavity is continuous with that 
of the elbow-joint, and therefore, in a sense, it lies within the cover of the capsule 
of the elbow-joint ; but its special feature is the— 
Orbicular ligament (lig. annulare radii, Figs. 211 and 214), which has formerly been 
Olecranon process mentioned as the inferior line of 
attachment of the external lateral 
ligament and the lgaments on 
the front and back of the elbow- 
TN. 
AS NW 
Lesser sigmoid 
cavity \ Transverse Joint. 
Je POnHONIO EY cae It is a strong well-defined struc- 
ligament ture, attached by its extremities 
_.. to the anterior and _ posterior 
Greater sigmoid : : : 
cavity margins of the lesser sigmoid 
cavity, and thus it forms nearly 
four-fifths of an osseo-tendinous 
circle orring. This circle is some- 
what wider at the upper than at 
the lower margin of the orbicular 
ligament, which, by encircling the 
upper part of the neck of the 
radius, tends to prevent displacement of the head of that bone in a downward 
direction. The lower margin of this ligament is not directly attached to the radius. 
The synovial membrane is continuous with that which lines the elbow-joint. It 
closes the joint cavity at the inferior unattached margin of the orbicular ligament, 
where it is somewhat loosely arranged in its reflexion from the ligament to the 
neck of the radius. 
Inferior Radio-ulnar Joint (articulatio radio-ulnaris distalis)—This joint is 
situated between the sigmoid cavity on the inner side of the lower end of the 
radius and the lateral aspect of the head of the ulna. In addition, it includes the 
inferior surface of the head of the ulna, which articulates with the superior 
surface of a triangular plate of fibro-cartilage, by means of which the joint is 
excluded from the radio-carpal articulation. 
The triangular interarticular fibro-cartilage (discus articularis, Figs. 215 and 217), 
besides presenting articular surfaces to two separate joints, is an important ligament 
concerned in binding together the lower ends of the radiusand ulna. It is attached 
by its apex to the de- 
pression at the outer 
side of the root of the 
styloid process of the 
ulna, and by its base to WS 
. . Surface for 
the sharp line of de- caphoid bone : 
sigmoid cavity and the Se ate Surface for semilunar bone 
carpal articular surface 
of the radius. 
The ligamentous cap- 
sule is very imperfect, and consists of scattered fibres, termed the anterior and 
posterior radio-ulnar ligaments (Fig. 216). These ligaments pass transversely 
between adjoining non-articular surfaces on the radius and ulna, and are of sufficient — 
length to pernut of the movements of the radius. * 
The synovial membrane completes the closure of the joint cavity. It forms a — 
a 
; 
7 
Orbicular ligament Coronoid process 
Fia. 214.—ORBICULAR LIGAMENT OF THE RADIUS. 
SOF 
Fig. 215.—Carpat ARTICULAR SURFACE OF THE RaDIUS, AND 
TRIANGULAR FIBRO-CARTILAGE OF THE WRIST. 
loose bulging projection (recessus sacciformis), passing upwards between the lower 
ends of the shafts of the radius and ulna, and it also clothes the upper surface of 
the triangular fibro-cartilage (Fig. 217). The cavity of this joint is quite distinct 
from that of the radio-carpal articulation, except when the triangular fibro-cartilage 
presents a perforation. 
Between the foregoing articulations there are two accessory ligaments, viz. the 
interosseous membrane and the oblique ligament, which connect together the shafts 
of the radius and ulna. 
