230 



RADIO-ULNAR ARTICULATIONS. 



of these articular facets so as to result in a 

 species of capsule. 



The triangular Jibro-cartilage is brought into 

 view by removing the preceding ligament 

 after laying open the wrist joint, and sepa- 

 rating the two bones. Arising by a broad 

 base from the sharp margin which separates 

 the ulnar and carpal articulating surfaces of 

 the radius, it passes inwards beneath the head 

 of the ulna with continually diminishing 

 width, until finally its apex is inserted into 

 the base of the styloid process of this bone. 

 At the commencement of this course it is 

 nearly flat, though rather thicker at the 

 margins than towards the middle ; indeed, it 

 is by no means unusual to find a " perfora- 

 tion " or deficiency in this part but towards 

 its apex its thickness is so much increased as 

 to give it almost a cord-like form where it 

 joins the ulna. It belongs to the class of 

 fibro-cartilages, and like most of these, the 

 proportions in which its component tissues 

 are mixed vary greatly in different parts: 

 thus the centre consists chiefly of cartilage, 

 while towards its periphery it is almost purely 

 ligamentous. Its lower surface is covered by 

 the synovial membrane of the carpal articu- 

 lation, and is in contact with the upper surface 

 of the cuneiform bone. Above, it corresponds 

 to the lower extremity of the ulna, and the 

 structure itself is the medium by which that 

 bone takes its limited share in the wrist joint. 

 Its borders, looking forwards and backwards, 

 are united with the anterior and posterior 

 ligaments of this articulation. 



The synovial membrane, " sacciformis," as it 

 is usually called, is large and loose, and is not 

 only interposed between the radial and ulnar 

 surfaces, but is also continued inwards beneath 

 the extremity of the ulna, so as to cover it 

 and the contiguous upper surface of the trian- 

 gular fib ro- cartilage. In passing from one of 

 these apposed surfaces to the other, it lines, 

 for a very short distance, the capsule and the 

 two ligaments of the wrist joint which unite 

 them. 



The movement of the lower end of the 

 radius may easily be deduced from the above 

 description, where the shape of the articular 

 surfaces and the attachments of the fibro- 

 cartilage alike indicate a rotatory movement 

 of this bone around the ulna ; since there is 

 an almost complete correspondence between 

 the apex of the ligament and the centre of 

 that circle of which these articular surfaces 

 would form a part. 



But although the motion at either of these 

 articulations is thus no very difficult deduction 

 from their anatomy, the mutual consistency 

 of the two, or the movement of the radius as 

 a whole, seems to have been much less un- 

 derstood. The somewhat obscure language 

 in which this has been described would allow 

 us to imagine that a kind of rotation of this 

 bone on its axis was supposed to result as 

 the balance of the movements which obtain at 

 the several joints. These anomalies and in- 

 consistencies have been cleared up by Mr. 

 Ward, in his very able work on Osteology; 



in which he points out that the axes of the 

 head and neck of the radius above, and that 

 of the head of the ulna below (the evident 

 centres of rotation in each case) are con- 

 tinuations of each other, and form different 

 portions of one and the same line, which is 

 thus the real axis of the whole bone in its 

 motions. In other words, the axis of the 

 head and neck of the radius, prolonged down- 

 wards, would fall upon a point in the lower 

 surface of the ulna, the centre of the circle 

 whereof the sigmoid cavity is a part. And 

 this, he urges, will alone explain how the 

 partial rotation of the bone is altogether in- 

 dependent of any antero-posterior movement 

 of its head, and occurs " without disturbance 

 to the parallelism of the superior joint." 



Thus we might imagine the articulations of 

 the forearm to be the immediate consequence 

 of two chief necessities of movement ; one of 

 flexion and extension of this segment of the 

 limb, another of alteration of aspect of the 

 terminal segment or hand ; the latter can 

 scarcely be accomplished in any other way 

 than by semirotation. The conditions of 

 powerful flexion and extension are, on the 

 contrary, best suited by a more or less gingly- 

 moid joint at each extremity; and the shape of 

 the interlocking surfaces which forms the chief 

 security of such an articulation, would render 

 it insusceptible of this partial rotation. These 

 requirements, incompatible of fulfilment by one 

 bone, are met by the addition of another, to 

 which the hand is attached. And now a new 

 necessity arises; for the superadded lever 

 must be associated with the pillar previously 

 existing, so far as regards the first movement, 

 but dissevered from it as regards the second. 

 This is accomplished by giving the radius a 

 very limited participation in the elbow joint, a 

 very considerable one in the wrist ; and by 

 making the ulna supply the terminal fixatures 

 of the rotating shaft. The peripheral and com- 

 plete condition of the upper attachment, the 

 internal or centric and incomplete state of the 

 lower, which, like the shaft itself, is here re- 

 duced to a part of a circle ; these are pro- 

 visions which, like many met with in other 

 parts of the body, at once economise means 

 and preserve the symmetry of the limb. 



Pronation and supination. The extremes 

 of this rotation of the lower extremity of the 

 radius constitute the states of pronation and 

 supination. So far as these result from the 

 movements of this bone, they are not quite 

 opposite aspects of its surfaces, or of those of 

 the hand, since the angles which they mutually 

 form in these conditions are scarcely equal to 

 a quadrant and a half, or 135 degrees. And 

 this fact, which the appearance of the articular 

 surfaces alone would lead us to suspect, may 

 be reduced to a certainty by the very simple 

 experiment of bending the forearm, and then 

 from extreme supination pronating the wrist, 

 and comparing the lines formed by its anterior 

 surface in both these positions with each other, 

 so as to take the angle through which the 

 surface has passed. Or better still, since it 

 removes all suspicion of interference with the 



