JOINTS OF TILE FINGERS.
1. Metacayo-phakngeat joints.—The first phalanges are articulated by an oval concave surface, with the rounded oblong heads of the inferior extremities of the metacarpal bones: it is remarkable that the long axis of the oval concavity of the phalanx has a transverse di rection, while the long axis of the head of the metacarpal bone is directed from before back wards, and consequently at right angles with the former ; whence the great extent of lateral motion enjoyed by these joints.
Each of these joints is strengthened by two lateral ligaments, of considerable strength, inserted into the tubercle behind the depression on each side of the head of the metacarpal bone; the point of insertion into the phalanx is anterior to this, and consequently the direc tion of the lateral ligaments is downwards and forwards; as they descend, these ligaments spread out, and their anterior fibres become identified with the anterior ligament.
A third ligament, the anterior ligament, or glenoid ligament of Cruveilhier, seems des tined more to increase the extent of the plia langeal articular concavity anteriorly, than to maintain the integrity of the joint or limit its motions. This ligament is, as Bichat expresses it, a thick and dense fibrous bundle, in shape half a ring, placed in front of the palmar sur face of the head of the metacarpal bone, com posed of transverse fibres which adhere in feriorly to the anterior edge of the concavity on the phalanx, and on each side are identified with the lateral ligaments and the transverse ligaments by which the metacarpal bones are connected. If the ligaments and synovial membrane of this joint be cut all round close to their attachment to the head of the meta carpal bone, and that bone be removed, the synovial capsule and ligaments remaining at tached to the phalanx, a very clear idea of the relative positions of the ligaments may be formed. The synovial membrane will then
appear protected on three sides by ligament ; on the radial and ulnar side by the lateral ligaments, and in front the anterior ligaments, whilst posteriorly it is unprotected save by the sheath of the extensor tendon.
In the metacarpo-phalangeal articulation of the thumb two sesamoid bones, developed in the substance of the anterior ligament, protect the joint in front.
2. Phalangeal joints.—These joints are all ginglymoid, the articular surfaces being pul ley-like ; they are provided with lateral liga ments similar to those of the metacarpo-pha langeal joints, and also with anterior ligaments similarly disposed.
Motions if the joints the fingers.—In the phalangeal joints these motions are only flexion and extension ; the former are con siderably more extensive, and are favoured by the inferior insertion of the lateral ligaments being on a plane anterior to their superior insertion. In addition to flexion and exten sion the metacarpo-phalangeal joints enjoy con siderable lateral motion, which is due to the glenoid form of the plmalangeal articular sur thee, and to the enarthrodial form which the joint derives from the extension of that arti cular surface by the anterior ligament.
(R. B. Todd.)