292 ARTICULATIONS. 



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 beneath 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 descends 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 with 

 the Flexor Longus Pollicis on the outer side, and with the Flexor Profundus 

 Digitorum on the inner, lying upon the interval between which are the anterior 

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

 ratus ; behind, with the Supinator Brevis, Extensor Ossis Metacarpi Pollicis, 

 Extensor Primi Internodii Pollicis, Extensor Secundi Internodii Pollicis, Ex- 

 tensor Indicis ; and, near the wrist, with the anterior interosseous artery and 

 posterior interosseous nerve. 



3. INFERIOR RADIO-ULNAR ARTICULATION. 



This is a lateral ginglymus, formed by the head of the ulna 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. 201) is a narrow band of fibres, 

 extending 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. 202) extends between similar points 

 on the posterior surface of the articulation. 



The Triangular Interarticular Fibro-cartilage (Fig. 203) 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 perforated. It is attached by its apex to a depression which sepa- 

 rates 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 mem- 

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

 under surface, by the synovial membrane of the wrist. 



The Synovial "Membrane (Fig. 203) of this articulation has been called, from 

 its extreme looseness, the membrana sacciformis; it covers the margin of the 

 articular surface of the head of the ulna, and where reflected from this bone on 

 to the radius, forms a very loose cul-de-sac ; from the radius, it is continued 

 over the upper surface of the fibro-cartilage. The quantity of synovia which 

 it contains is usually considerable. When the_fibro-cartilage is perforated, the 

 synovial membrane is continuous with that which lines the wrist. 



