2i2 THE SUPERIOR EXTREMITY 



of the synovial stratum only, the protection afforded by the 

 extensor tendon rendering the presence of the fibrous stratum 

 unnecessary. 



Movements of the Metacarpo-phalangeal Joints. In each metacarpo- 

 phalangeal joint the single concavity at the proximal end of the first 

 phalanx articulates with the rounded distal extremity or head of the 

 metacarpal bone. The joints are condyloid joints, therefore the move- 

 ments which occur at them are: (i) flexion, (2) extension, (3) abduc- 

 tion, (4) adduction, (5) circumduction. 



During flexion of the fingers the first phalanx travels volarwards with 

 the thick accessory volar ligament upon the head of the metacarpal 

 bone. The interosseous and lumbrical muscles are chiefly instrumental 

 in producing this movement, but they are aided by the long and short 

 flexors of the digits. 



The first phalanges of the fingers, in the movement of extension, can 

 be carried dorsally only to a very slight degree beyond the line of the 

 metacarpal bones. The extensor communis and the special extensors of the 

 index and little finger are the muscles which operate in this case. 



Abduction and adduction are movements of the first phalanx away from 

 and towards a line prolonged distally through the middle finger, and 

 are seen when the fingers are spread out and again drawn together. 

 The abductor digit i quint i and the dorsal interosseous muscles act as 

 abductors of the fingers, whilst the volar interosseous muscles operate 

 as adductors of the little, ring, and index fingers. In the case of 

 the middle digit, the second and third dorsal interosseous muscles act 

 alternately as abductors and as adductors. In connection with the 

 movements of abduction and adduction, it should be noticed that in the 

 extended position of the fingers they are very free ; but if flexion is 

 induced, the power of separating the fingers becomes more and more 

 restricted, until it becomes lost when the hand is closed. An examina- 

 tion of the collateral ligaments will afford the explanation of this fact. 

 They " are attached so far dorsally on the metacarpal bones, as to be much 

 nearer to their distal ends than to their volar aspects " (Cleland). Con- 

 sequently, while they are comparatively lax in the extended position of 

 the fingers, the further flexion advances the tighter they become, and in 

 this way they interfere with the lateral movements of the first phalanges. 



The first phalanx of the thumb has only a limited range of movement 

 at the metacarpo-phalangeal joint. 



Movements of the Interphalangeal Joints. The inter phalangeal 

 joints are hinge joints, therefore the only movements possible are flexion 

 and extension. Flexion of the second phalanges of the fingers is brought 

 about by the flexor sublimis, and of the ungual phalanges by the flexor 

 profundus. Extension of the phalanges at the interphalangeal joints is 

 produced not only by the extensors of the digits but also by the interosseous 

 and lumbrical muscles acting through the extensor expansions, into which 

 they are inserted. The interossei and lumbricals, therefore, flex the first 

 phalanges at the metacarpo-phalangeal joints and extend the second and 

 ungual phalanges at the interphalangeal joints. 



In the case of the thumb, the flexor pollicis longus and the extensor 

 pollicis longus operate at the interphalangeal joint. 



Some of the metacarpo-phalangeal joints and interphalangeal joints are 

 supplied by the median and some by the ulnar nerve. 



