METACARPO-PHALANGEAL JOINTS 275 



transverse metacarpal ligament. It corresponds to the sesamoid bones of the thumb; a sesa- 

 moid bone sometimes exists at the medial border of the Joint of the little finger. 



The collateral ligaments (304 and 305) are strong and firmly connect the bones with one 

 another; each is attached above to the corresponding tubercle, and to a depression in front 

 of the tubercle, of the metacarpal bone. From this point the fibres spread widely as they de- 

 scend on either side of the base of the phalanx; the anterior fibres are connected with the glenoid 

 ligament; the posterior blend with the tendinous expansion at the back of the joint. 



The joint is covered in posteriorly by the expansion of the extensor tendon, and some loose 

 areolar tissue passing from its under surface to the bones (fig. 305). 



The synovial membrane is loose and capacious, and invests the inner svu-face of the hga- 

 ments which connect the bones. 



The arteries come from the digital or volar metacarpal vessels of the deep arch. 



The nerves are derived from the digital branches, or from twigs of the branches of the ulnar 

 to the interosseous muscles. 



Relations. — I. The metacarpo-phalangeal joints of the middle three digits. In front, 

 the tendons of the flexor profundus and flexor subhmis digitorum. On the radial side, a lum- 

 brical, an interosseous muscle, and digital nerves and vessels; on the ulnar side, an interosseous 

 muscle and digital vessels and nerves. Behind, the common extensor tendon and in the case 

 of the index digit the tendon of the extensor indicis. 



II. The metacarpo-phalangeal joint of the Uttle finger. In front, the flexor quinti digiti 

 brevis and the tendons of the flexor profundus and subhmis digitorum muscle which go to 

 this digit. Behind, the extensor digiti quinti to a slip of the extensor digitorum communis 

 sometimes. On the radial side, a lumbrical, the third palmar interosseous muscle, digital ves- 

 sels and nerves. On the ulnar side, digital vessels and nerves. 



The movements permitted at these joints are flexion, extension, abduction, adduction, and 

 circumduction. Flexion is the most free of all and may be continued until the phak nx is at 

 a right angle with the metacarpal bone. It is on this account that the articular surface of the 

 head of the bone is prolonged so much fm-ther on the palmar aspect, and that the synovial 

 membrane is here so loose and ample. Extension is the most Umited of the movements, and can 

 only be carried to a little beyond the straight line. Abduction and adduction are fairly free, 

 but not so free as flexion. Flexion is associated with adduction, and extension with abduction. 

 This may be proved by opening the hand, when the fingers involuntarily separate as thej'' 

 extend, while in closing the fist they come together again. The free abduction, adduction, and 

 circumduction which are permitted at these joints are due to the fact that the long axes of the 

 articular facets are at right angles to one another. 



Muscles acting on the middle three digits. — Flexors. — Flexor digitorum profundus, flexor 

 digitorum subhmis. Extensors. — Extensor digitorum communis and on the index digit the 

 extensor indicis. Abductors. — Dorsal interossei. Adductors. — Volar interossei. 



Muscles acting on the metacarpo-phalangeal joint of the little finger. — Flexors. — Flexor 

 quinti digiti brevis, flexor digitorum sublimis, flexor digitorum profundus. Extensors. — Exten- 

 sor digitorum communis, extensor quinti digiti. Abductor. — Abductor quinti digiti. Adductor. 

 — Third volar interosseous. 



(6) The Metacarpo-phalangeal Joint of the Thumb 

 Class. — Diarthrosis. Subdivision. — Condylarthrosis. 



The head of the metacarpal bone of the thumb differs considerably from the 

 corresponding ends of the metacarpal bones of the fingers. It is less convex, wider 

 from side to side, the palmar edge of the articular surface is raised and irregular, 

 and here on either side of the median line are the two facets for the sesamoid 

 bones. The base of the first phalanx of the thumb, too, is more hke the base of 

 the second phalanx of one of the other fingers. The ligaments are: — 



Collateral. Dorsal. 



Articular capsule. 



The collateral ligaments are short, strong bands of fibres, which radiate from depressions 

 on either side of the head of the metacarpal bone to the base of the first phalanx and sesamoid 

 bones. As they descend they pass a Httle forward, so that the greater number are inserted in 

 front of the centre of motion. 



The dorsal ligament consists of scattered fibres which pass across the joint from one col- 

 lateral ligament to the other, completing the articular capsule and protecting the synovial sac. 



The sesamoid bones are two in number, situated on either side of the middle line, and con- 

 nected together by strong transverse fibres which form the floor of the groove for the long 

 flexor tendon; they are connected with the base of the phalanx and head of the metacarpal bone 

 by strong fibres. Anteriorly they give attachment to the short muscles of the thumb, and pos- 

 teriorly are smooth for the purpose of ghding over the facets. The collateral hgaments are partly 

 inserted into their sides. 



The arteries and nerves come from the digital branches of the thumb. 



Relations. — Of the metacarpo-phalangeal joint of the thumb: In front and externally 

 abductor poUicis brevis and superficial head of flexor poUicis brevis. In front and medially 

 obhque and transverse adductors and deep head of flexor poUicis brevis. Directly in front 

 flexor poUicis longus and terminal branches of first volar metacarpal artery. Behind, extensor 

 poUicis brevis and longus tendons. On either side, the dorsal digital vessels and the digital 

 nerves. 



