Limbs: Upper Limb 115 



of phalanx : it has some attachment to these and to the interosseous tendons, so that 

 the direct action of the extensor tendon on the finger is mainly to extend the first 

 phalanx through this attachment. 



Observe that the articular surface in the metacarpo-phalangeal joint of the thumb 

 is flatter and broader but less extensive from before backwards than those of the other 

 bones : associated with this is the much lessened range of flexion and practical absence 

 of power of lateral movement in this joint. Its ligaments are the same with the excep- 

 tion of the transverse metacarpal band : the sesamoids are large and always present 

 and the comparative slightness of the grooves for them depends on the smaller extent 

 of the articular surface. 



PHALANGES. 



The distinguishing characters of these bones have already been noticed (p. 105;. 

 Looked at from behind the phalanges are rounded and cylindrical in appearance, 

 enlarged at their ends, but on their palmar surfaces they are flattened and bounded 

 by sharp margins on each side. The rounded dorsal surfaces are covered by the expan- 

 sions of the extensor tendons, while the flat, anterior surfaces form the bony bed of 

 the flexor tendons, against which they are held by the fibrous sheath that is fastened 

 to the margins of the bones. 



The first phalanx is attached to the metacarpal as already described, and the inter- 

 phalangeal joints are formed on the same plan, save that there is of course no represen- 

 tative of the transverse metacarpal band. Thus we find rounded facets on the sides 

 of the distal extremities for the lateral ligaments and ligamentous markings on the 

 proximal ends for these and for the capsules in front. 



The presence of condyles with intervening depressions in the interphalangeal 

 articulations indicates that there can be no lateral movements in these joints, and the 

 lateral ligaments are attached at what is practically the centre of movement in flexion 

 and extension, so that they are constantly tense. 



The dorsal expansion covers the articulations behind and is attached to the bases 

 of the last two phalanges, making a well-marked roughness there, which is therefore 

 not ligamentous. 



The theca which holds down the flexor tendons, is not a continuous fibrous layer, 

 but is made up of bands (Fig. 94), so that the markings for it on the bones are irregular. 

 On the first phalanx they are on the margins : the lines of attachment then cross the 

 capsule and lateral ligaments to reach the margins of the second phalanx, then pass 

 on to the third where they meet, more or less fused with the rough basal ridge that 

 marks the insertion of the Flexor profundus tendon. The markings for the different 

 bands that compose the sheath cannot be recognised with any certainty on the bone, 

 but it can be noticed that the upper crucial fibres cause a roughness on the first phalanx 

 behind the line of its margin. 



Between the margins the bones are covered by the synovial sheath of the tendons, 

 and on the second phalanx the markings of insertion of Flexor sublimis are easily seen, 

 distinct from the marginal sheath marking, opposite the middle and proximal parts of 

 the shaft. 



Where the capsules of the interphalangeal joints support the tendons they are 

 thickened into a single sesamoid body which occupies the anterior notch between the 

 condyles of the phalanges : this raises the tendons slightly from the bones to which 

 they are going, and so far increases their power of action on these bones. 



82 



