RADIO-ULNAR JOINTS 



263 



process, downward and laterally to be attached to the posterior edge of the 

 lower end of the tuberosity of the radius and the vertical ridge running from it 

 to the medial border of the bone. 



Some of its fibres blend with the fibres of insertion of the biceps tendon; behind, it is in 

 close contact with the supinator; below, a thin membrane passes off from it to the upper edge 

 of the interosseous membrane; the posterior interosseous vessels pass in the space between it 

 and the interosseous membrane; occasionally a slip is continued into the annular Hgament of the 

 superior radio-ulnar articulation (see fig. 298). 



The interosseous membrane (fig. 293) is attached to the ulna at the lowest 

 part of the ridge in front of the depression for the supinator, and along the whole 

 length of the interosseous border as far as the inferior radio-ulnar articulation, 

 approaching the front of the bone in the lower part of its attachment. To the 

 radius it is attached along the interosseous border, from an inch (2.5 cm.) below 

 the tuberosity to the ulnar notch for the lower end of the ulna. 



It is strongest and broadest in the centre, where the fibres are dense and closely packed ; 

 it is also well marked beneath the -pronator quadratus, and thickens considerably at the lower end,- 

 forming a strong band of union between the two bones. Its fibres pass chiefly downward 

 and medially, from the radius to the ulna, though some take the opposite direction; at the lower 

 end some are transverse. On the posterior surface are one or two bands, which pass downward 

 and laterally from the ulna to the radius, and frequently there is a strong bundle as large as the 



Fig. 298. — Upper Portions of Left Ulna and Radius, to show an Occasional Slip from 

 THE Oblique Cord to the Lower Part of the Annular Ligament. This condition is 

 present in the spider monkey {Ateles), which has no external thumb but only rudimentary 

 bones of one. 



(From a dissection by Mr. W. Pearson, Royal CoUege of Surgeons, England.) 



Annular ligament 



Occasional slip from oblique cord 



to annular ligament ■!>. 



Ulna 



B a dins / , -Z — ! 1 Oblique cord 



I /A- f I, iif\ 



oblique cord; this, which may be called the inferior oblique ligament (fig. 303), stretches from 

 the ulna, an inch and a half above its lower extremity, downward and laterally to the ridge 

 above and behind the ulnar notch of the radius. 



At its attachment to the bones, the interosseous membrane blends with the periosteum. 

 Its upper border is connected with the obUque cord by a thin membrane, which is pierced by 

 the posterior interosseous vessels; and the lower border, which stretches across between the two 

 bones just above the inferior radio-ulnar articulation, assists in completing the capsule of that 

 joint. Its anterior surface is in relation with the flexor digitorum profundus and flexor pollicis 

 longus in the upper three-quarters, the lower fourth being in relation with the pronator quadratus. 

 The anterior interosseous vessels and nerve descend along the middle of the membrane, the 

 artery being bound down to it. About an inch from the lower end it is pierced by the anterior 

 interosseous artery. The posterior surface is in relation with the supinator, abductor pollicis 

 longus {extensor ossis metacarpi pollicis), extensor pollicis longus and brevis, and the extensor 

 indicis proprius; at its lower part, also with the posterior branch of the anterior interosseous 

 artery, and the deep branch of the radial nerve (posterior interosseous). 



(c) The Inferior Radio-ulnar Joint 



Class. — Diarthrosis. Subdivision. — Trochoides. 



This is, in one respect, the reverse of the superior; for the radius, instead of 

 presenting a circular head to rotate upon the facet on the ulna, presents a concave 

 facet which rolls round the ulna. The articulation may be said to consist of two 



