ii4 Anatomy of Skeleton 



The radial flexor is partly fused with the ligaments in its attachment to the middle metacaipal. 

 The main insertion of Ext. c. rad. brevior is on to the base of the styloid process, but a contiguous 

 impression on the index metacarpal receives part of it : bursae lie under the two radial extensor 

 tendons at their insertions. The ulnar flexor is not directly inserted into the metacarpus, but the 

 piso-metacarpal ligaments may be looked on as a continuation of its tendon to the inner bones. 



The Ulnar extensor may have a small bursa between its tendon and the proximal part of the 

 tubercle on the inner and dorsal aspect of the base of the fifth metacarpal. 



The heads are somewhat square-cut, rounded off in an antero-posterior direction. 

 Observe that the articular surfaces are slightly more vertically extensive in front than 

 behind in addition to being broader, and that they show a transverse constriction 

 owing to the existence of the depressions on the sides of the head : the constriction 

 is best marked in the index metacarpal and least apparent in the fifth. Each surface 

 is prolonged proximally in front and at the sides into two cornua, on which the sesa- 

 moid bones or cartilages play that are found in the palmar capsular structures 

 (see later). 



These sesamoids are always present and well developed in the thumb, and frequently, if not 

 usually, in the index finger on the outer side and in the little finger on the inner side, and corre- 

 sponding differences in the size of the cornua can be noticed in the metacarpals. The sesamoids are 

 more numerous in the foetus, especially on the radial side of the index and middle fingers, and on 

 the ulnar side in the ring and little fingers. 



The depressions on each side of the head have a facetted surface behind them, 

 and to this also the lateral ligament of the joint is attached. The anterior part of the 

 capsule is thickened and firmly attached to the base of the phalanx, but only by areolar 

 tissue to the metacarpal, so that there is no very definite marking to be found for it on 

 these bones. 



The extensor expansion covers in the posterior aspect of the joint and takes the 

 place of a capsule there. Thus the head of each bone may be said to be surrounded 

 by ligamentous and fibrous structures : these are all connected together, but the mass 

 can be analysed into its constituent parts as shown in Fig. 94. 



1. The deepest structure is the proper capsule, which extends on the palmar 

 aspect of the joint from one lateral ligament to the other : it has the sesamoid bones 

 or cartilages when present embedded in it, and is firmly attached to the base of the 

 phalanx, but lies loosely on the metacarpal, so that it does not limit extension of the 

 joint. 



Each lateral ligament has an extensive attachment to the metacarpal (Fig. 94) and 

 the phalanx, so that its front fibres become tight in extension and its posterior part in 

 flexion : the interosseous tendons are partly attached to the lateral ligaments. 



2. The transverse metacarpal ligament lies on the palmar surfaces of the capsules, 

 a strong fibrous band stretching across the four inner joints and only indirectly 

 attached by their capsules to the bones : its function is evidently to limit divergence 

 of the bones. 



The interosseous tendons lie behind it, while the plane of the Lumbricales and 

 digital vessels and nerves is in front of it. 



3. The proximal end of each digital sheath (theca) lies on the transverse band, 

 and thus thickens still more the structures in front of the joint. 



The vessels and nerves pass down between the commencement of the sheaths. 



4. The extensor expansion throws a fibrous covering downwards and forwards on 

 each side of the joint, covering it in and passing over the lateral ligaments and base 



