RADIO-ULNAR ARTICULATIONS. 



229 



but it is to the latter only that our attention 

 is at present directed. 



The sigmoid cavity of the ulna is a depres- 

 sion situated on the outer side of its upper ex- 

 tremity, and, in respect of its position, it 

 might be expressed as an articular facet seated 

 on the external margin of the coronoid process. 

 In shape it is somewhat quadrilateral ; and is 

 concave in both directions, but most so in the 

 anterio-posterior, which corresponds to the 

 convexity of the head of the radius, and is 

 also much the longest surface of the two. 

 With trifling individual variations, it usually 

 forms about the fourth of a circle. Superiorly, 

 it is separated from the greater sigmoid cavity 

 by a smooth elevation directed from before 

 backwards : anteriorly, inferiorly, and pos- 

 teriorly, the border of this articular surface 

 overhangs the coronoid process of the ulna, 

 the concave upper part of its anterior surface, 

 and its posterior surface respectively. The 

 junction of the two latter sides of its margin 

 is marked by a strong ridge, which commences 

 the external border of the bone: and, fre- 

 quently the antero-inferior angle gives off a 

 similar prominence ; which, after a short 

 course downwards, converges to join the 

 preceding. 



Articular cartilage covers these surfaces of 

 the radius and ulna. 



The annular or orbicular ligament is the 

 next constituent, and is a strong and some- 

 what cord -like band of white fibrous tissue, 

 which completes the remaining three -fourths 

 of the articular circle left unaccounted for by 

 bone. Its width is about one third of an inch, 

 its direction is horizontal like that of the sig- 

 moid cavity. It arises behind from the poste- 

 rior margin of this surface, and partly from its 

 inferior border, uniting beyond these" with the 

 periosteum covering the surfaces of bone over- 

 hung by them. In front, it is inserted into 

 the anterior margin in a similar manner. 

 Above, it receives and is continuous with the 

 anterior ligament of the elbow joint ; far- 

 ther outward, it is also joined by the external 

 lateral ligament of the same articulation. Its 

 lower border is free around the neck of the 

 radius. 



The synovial membrane is a process sent off 

 from that which lines the articular surfaces of 

 the elbow joint. A cul-de-sac passes down- 

 wards into the lesser sigmoid cavity, extending 

 to its inferior extremity, but around the neck 

 of the radius, and between it and the orbicular 

 ligament, the remainder of this circular pouch 

 has a diminished vertical extent ; sufficient, 

 however, to allow it to pass under the or- 

 bicular ligament, and appear from beneath its 

 lower border. 



The movement of the head of the radius at 

 this articulation is one of simple rotation 

 around its own axis ; since the articular sur- 

 faces in contact with it together form a circle, 

 in which its only movement can be a revo- 

 lution. And, as above stated, about three- 

 fourths of this circle is formed by ligament j 

 the remainder by bone. But in addition to 

 this chief provision for the limitation and di- 



rection of motion, the convex radial tuberosity 

 of the humerus forms a kind of pivot, which 

 is received into the cavity which occupies the 

 upper surface of the radius, and, no doubt, 

 steadies and assists the movement by tending 

 still more to define the axis of this part of the 

 bone. The articulation of the atlas with the 

 odontoid process of the axis, offers many 

 analogies to this of the radius and ulna both 

 in the structure of the joint and in the re- 

 sulting movements. 



(2.) The lower radio-ulnar articulation is, 

 in many respects, the reverse of the preceding ; 

 since instead of presenting a cylindrical ex- 

 tremity of the radius revolving within a con- 

 cave facet of the ulna, the latter bone itself 

 offers a rounded termination, on and around 

 the outer side of which the radius plays by a 

 concave articular surface. The constituents of 

 the joint are, the surfaces of the radius and ulna 

 just alluded to ; a fibro-cartilage which, with a 

 kind of imperfect ligamentous capsule, forms 

 the means of union of the bones ; and a syno- 

 vial membrane interposed between their ar- 

 ticular surfaces. 



The loiver extremity of the radius ap 

 preaches somewhat to the form called by 

 geometricians a parallelepiped. Its largest sur- 

 faces are the anterior and the posterior : the 

 upper is joined and surmounted by the shaft 

 of the bone, and the lower enters into the 

 formation of the wrist joint. The outer side 

 is occupied by the tendons of the muscles 

 which extend the thumb : and the inner, 

 which looks slightly upwards, articulates with 

 the ulna. 



This surface is quadrilateral, and of these 

 the two antero-posterior sides are much the 

 longest. The upper is nearly straight, the 

 lower somewhat concave downwards to adapt 

 it to the convex surface of the radio-carpal 

 articulation ; and they slightly diverge behind 

 so as to make the posterior vertical border 

 almost twice as deep as the corresponding 

 anterior side. The articular surface itself is 

 concave from before backwards, taking a curve 

 whose extent is about one fifth of a circle. 



The lower end or head of the ulna is of 

 even smaller size than the upper extremity of 

 the radius which was previously described ; 

 a condition which is in conformity with its 

 slight share in the wrist joint. The base of 

 this cylindrical head has a smooth surface 

 and is almost circular in shape ; internally 

 it offers a depression bounded by the pro- 

 minent styloid process extending vertically 

 downwards; externally, a margin defines its 

 separation from the articular facet which oc- 

 cupies the outer part of the cylinder. 



This convex surface is usually a little longer 

 in the horizontal direction than the corre- 

 sponding radial concavity, forming about a 

 fourth of a circle ; but in all other respects it 

 is, as it were, moulded to it. Above, its 

 margin projects beyond the constricted shaft. 

 A layer of articular cartilage covers both 

 these surfaces. Ligamentous fibres in sparing 

 quantities, and with no very definite direction, 

 unite the upper, anterior, and posterior borders 



Q 3 



