THE INTERPHALANGEAL 245 



The lateral 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 })halanx and sesamoid bones. As they descend they pass a little forwards, so 

 that the tri-eater nuinl)er are inserted in front of the centre of motion. 



The posterior ligament consists of scattered fibres which pass across the joint 

 from one lateral ligament to the other, completing the capsule and protecting the 

 synovial sac. 



The sesamoid bones are two in number, situated on either side of the middle 

 line, and connected 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 attach- 

 ment to the short muscles of the thumb, and posteriorly are smooth for the purpose 

 of gliding over the facets. The lateral ligaments are partly inserted into their sides. 



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



The movements are chiefly flexion and extension, very little lateral movement 

 being permitted, and that only when the joint is slightly bent. Thus this joint 

 more nearly approaches the simple hinge character than the corresponding articu- 

 lations of the fingers. The thumb gets its freedom of motion at the carpo- 

 metacarpal joint; the fingers get theirs at the metacarpo-phalangeal, but they are 

 not endowed with so much freedom as the thumb enjoys. 



11. THE INTERPHALANGEAL ARTICULATIONS 

 Class, — Diarthrosis. Subdivision. — G'mglynms. 



The ligaments which unite the phalanges of the thumb and of the fingers are : — 

 Glenoid. Lateral. 



The glenoid ligament (fig. 236), sometimes called the sesamoid body, is very 

 firmly connected with the base of the distal bone, and loosely, by means of fibro- 

 areolar tissue, with the head of the proximal one. It blends with the lateral 

 ligaments at the sides, and over it pass the flexor tendons. Occasionally a sesamoid 

 bone is developed in the cartilage of the interphalangeal joint of the thumb. 



The lateral ligaments (figs. 235 and 236) are strong bands which are attached 

 to the rougli depressions on the sides of the upper phalanx, and to the projecting 

 lateral margins of the lower phalanx of each joint. They are tense in every 

 position, and entirely prevent any lateral motion; they are connected posteriorly 

 with the expansion of the extensor tendon. 



Posteriorly (fig. 236) the joint is covered in by the deep surface of the extensor 

 tendon, and a little fibro-areolar tissue extends from the tendon, and thickens the 

 posterior portion of the synovial sac, completing the capsule. 



The synovial membrane is loose and ample, and extends upwards a little way 

 along the shaft of tlie proximal bone. 



The arteries and nerves come from their respective digital branches. 



The movements are limited to flexion and extension. Flexion is more free, 

 and can be continued till one bone is at a right angle to the other, and is most free 

 at the junction of the first and second bones; the second phalanx can be flexed on 

 the first through 110° to 115° when the latter is not flexed. The greater freedom 

 of flexion is due to the greater extent of the articular surface in front of the heads 

 of the proximal bones, and to the direction of the fibres of the lateral ligaments, 

 which ])ass a little forwards to their insertion into the distal bone. 



