92 Anatomy of Skeleton 



it in constant contact, yet do not in the least impair its power of rotation round that 

 bone : these can be co nsidered under three headings : 



(1) The interosseous membrane and oblique ligament. 



(2) The lower radio-ulnar joint, especially the triangular fibro-cartilage. 



(3) The upper joint : the orbicular ligament. 



(1) Interosseous Membrane. Aponeurotic, becoming thin and fascial at its lower 

 end. Fibres chiefly directed downwards and inwards, from radius to ulna : some 

 bands, however, on posterior aspect running in the opposite direction. The inte- 

 rosseous borders on the bones fade away below, showing a tendency to reach the pos- 

 terior aspect : here the thin membrane becomes continuous with the annular and other 

 ligamentous structures on the back of the wrist. 



It has a free edge above, corresponding with the upper border of Extensor ossis 

 metacarpi pollicis, and the posterior interosseous artery passes over this. The anterior 

 interosseous artery pierces it near the lower fourth. 



The main function of this sheet is to afford additional surface for the origin of deep 

 flexor and extensor muscles, but, owing to the direction of the majority of its fibres, it 

 can also, when taut, transmit to the ulna some of the pressure that is more directly exerted 

 on the radius through the hand and at the same time will tend to keep the bones together 

 in proportion as the pressure increases. That this is not the chief function of the 

 membrane, however, is evident when we find that it is not taut during pronation, which 

 is the position most commonly assumed when pressure is transmitted from the hand. 



The oblique ligament : not a part of the interosseous membrane : a band directed 

 down and out from the outer margin of the coronoid process to the radius just below 

 its tubercle, and therefore slightly in front of the plane of interosseous membrane 

 as well as altogether above it. On the ulna it is, when broad, continuous at its upper 

 end with the orbicular ligament, but it is as a rule a feeble band ; its mechanical action, 

 if of any value at all, would be to check any downward sliding of radius on ulna. 



(2) Fibro-Cartilage of Lower Radio-Ulnar Joint. This is responsible for keeping 

 the bones apposed at their lower ends, for the feeble ligaments of this joint are of 

 necessity very lax to permit of the extensive rotation of the radius round the head of 

 the ulna. The structure is triangular in shape, the apex fastened to the ulna at the 

 outer side of the base of the styloid process, in the axis of rotation, and the base attached 

 to the inner margin of the lower end of radius : it follows, therefore, that the radius 

 can rotate round its axis, made fast to it by the fibro-cartilage, which is thus like a 

 broad spoke, so to speak, in the wheel in the circumference of which the radius moves. 

 It is usually only definitely cartilaginous in its outer part. 



Synovial membrane is loose like the capsule, and is sometimes termed the Mem- 

 brana sacciformis. The capsule is attached to the edges of the fibro-cartilage, so that, 

 unless this is perforated, the radio-ulnar cavity is quite separate from the radio-carpal 

 joint. 



(3) The Orbicular Ligament holds the head of the radius in the lesser sigmoid 

 cavity of the ulna : it is a part of the general capsule of the elbow-joint, from which 

 it cannot be separated (Fig. 72). 



Attached to the front and back margins of the lesser cavity, the orbicular fibres 

 surround the radial head, to which they are not fastened, so that it is free to rotate 

 within the circle formed by them and the cavity. It is lined by synovial membrane 

 which is reflected from its lower border on to the neck of the radius. The fibres not 

 only surround the head, but also the upper part of the neck, and here they are somewhat 

 constricted so that the upper end of the bone is more firmly grasped by them. 



