214 ARTICULATIONS. 



lesser sigmoid cavity of the ulna ; rotation forwards being called pronation ; rota- 

 tion backward, supination. 



2. MIDDLE KADIO-ULNAR ARTICULATION. 



The interval between the shafts of the radius and ulna is occupied by two liga- 

 ments. 



Oblique. Interosseous. 



The Oblique or Round Ligament (fig. 132) is a small round fibrous cord, which 

 extends obliquely downwards and outwards, from the tubercle of the ulna at the 

 base of the coronoid process, to the radius a little below the bicipital tuberosity. 

 Its fibres run in the opposite direction to those of the interosseous ligament ; and 

 it appears to be placed as a substitute for it in the upper part of the interosseous 

 interval. This ligament is sometimes wanting. 



The Interosseous Membrane is a broad and thin plane of aponeurotic fibres, 

 descending obliquely downwards and inwards, from the interosseous ridge on the 

 radius to that on the ulna. It is deficient above, commencing about an inch be- 

 neath the tubercle of the radius ; is broader in.the middle than at either extremity ; 

 and presents an oval aperture just above its lower margin for the passage of the 

 anterior interosseous vessels to the back of the forearm. This ligament serves to 

 connect the bones, and to increase the extent of surface for the attachment of the 

 deep muscles. Between its upper border and the oblique ligament an interval 

 exists, through which the posterior interosseous vessels pass. Two or three fibrous 

 bands are occasionally found on the posterior surface of this membrane, which 

 descend obliquely from the ulna towards the radius, and which have consequently 

 a direction contrary to that of the other fibres. It is in relation, in front, by its 

 upper three-fourths (radial margin) with the Flexor longus pollicis (ulnar margin), 

 with the Flexor profundus digitorum, lying upon the interval between which are 

 the anterior interosseous vessels and nerve, by its lower fourth with the Pronator 

 quadratus ; behind, with the Supinator brevis, Extensor ossis metacarpi pollicis, 

 Extensor primi internodii pollicis, Extensor secundi internodii pollicis, Extensor 

 indicis ; and, near the wrist, with the anterior interosseous artery and posterior 

 interosseous nerve. 



3. INFERIOR KADIO-ULNAR ARTICULATION. 



This is a lateral ginglymoid joint, formed by the head of the ulna being received 

 into the sigmoid cavity at the inner side of the lower end of the radius. The 

 articular surfaces are covered by a thin layer of cartilage, and connected together 

 by the following ligaments. 



Anterior radio-ulnar. 



Posterior radio-ulnar. 



Triangular Interarticular Fibro-cartilage. 



The Anterior Radio-ulnar Ligament (fig. 134) is a narrow band of fibres, ex- 

 tending from the anterior margin of the sigmoid cavity of the radius to the anterior 

 surface of the head of the ulna. 



The Posterior Radio-ulnar Ligament (fig. 135) extends between similar points 

 on the posterior surface of the articulation. 



The Triangular Fibro-cartilage (fig. 136) is placed transversely beneath the 

 head of the ulna, binding the lower end of this bone and the radius firmly together. 

 Its circumference is thicker than its centre, which is thin and occasionally per- 

 forated. 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, is contiguous with the head 

 of the ulna ; its under surface, also concave and smooth, with the cuneiform bone. 

 Both surfaces are lined by a synovial membrane : the upper surface, by one peculiar 



