The inferior surface of the radius presents a triangular outline, the apex of which is placed externally at the styloid process, whilst the base, in the opposite direction, is consti tuted by the sharp margin separating the inferior from the internal articular surface on the radius, and serving for the attachment of the broad extremity of the radio-ulnar inter articular cartilage (triangular fibro cartilage). The two margins which border this inferior articulating surface of the radius serve for the attachment of ligaments, and of these the anterior is the most prominent. Lastly, the carpal surface of the radius is divided into two portions by a ridge, which traverses it from before backwards : the external of these articular " facettes " is triangular in shape, and adapted to the scaphoid bone, whilst the more internal of the two is quadrilateral, and articulates with the semilunar carpal bone.
It has been mentioned that the radius at its lower extremity undergoes a change of form, and that from being cylindrical higher up, it becomes here quadrilateral. This ex pansion of its surface takes place at the expense of its solidity, for a section of the bone shows that inferiorly the compact tissue is extremely thin, whilst the cancellated tissue is in proportion more abundant.
This circumstance is adduced, as in some measure explaining the frequency of fracture in this situation, an accident notoriously common.
The ulna is excluded from the articulation of the wrist by the triangular fibro-cartilage, which stretches across transversely between the head of the ulna above and the carpus inferiorly, and presents a surface concave in each direction.
The inferior surface of this fibro-cartilage is on the stone plane with the inferior surface of the radius, and constitutes with it the superior articulating surface of the wrist-joint. The entire of this conjoint surface is some what oval (or diamond-shaped), and is limited externally and internally by the styloid pro cesses of the radius and ulna respectively.
b. The Scaphoid, Semilunar, and Cuneiform Bones of the Carpus.— The Carpus, superiorly, presents a surface which is pretty uniformly convex. That convexity, however, is slightly interrupted by the undulating lines resulting from the lateral articulations between the bones which compose it.
The convexity of the upper surface of the scaphoid bone is triangular in form, and articu lates with the outer facette on the inferior surface of the radius; the semilunar bone is quadrilateral in conformity with the shape of the inner facette of the radius, whilst the cuneiform bone presents a surface of a trian gular form, by which it comes accurately into apposition with the 'inferior surface of the radio-ulnar interarticular cartilage. The aspect
of the radial or upper conjoint surface of those three bones is directed upwards and slightly backwards.
II. The These are placed on each of the several aspects of the wrist, and are usually designated from their position,— the anterior, the posterior, and the lateral radio-carpal ligaments. But whilst the wrist joint is thus protected on its different aspects by ligamentous bands, it is to be observed that no distinct intervals naturally separate these ligaments from one another, so that it might with strict propriety be said that the articulation of the wrist is defended by a ligamentous envelope of a capsular form, strengthened in particular parts, but more especially laterally by other superimposed and superadded fibres.
a. Anterior Ligament. — The fibres of this ligament are connected superiorly to the anterior margin of the lower end of the radius and of its styloid process, and to the " triangular ligament," from whence they radiate to their insertion into the anterior surfaces of the scaphoid, semilunar, and cunei form bones. Below, they are partly con tinuous with the fibrous fasciculi (interosseous ligament), which connect the two rows of carpal bones.
The fibres of the anterior radio-carpal ligament pursue a direction for the most part downwards and inwards.
By its anterior surface this ligament is in relation with the tendons of the deep flexor muscles, and also (but more remotely), with those of the superficial flexor, and with the median nerve, the entire surrounded by an extensive and complicated synovial apparatus. Beneath these structures, and immediately in front of the ligament, is the anastotnosis, between the anterior radial and the anterior ulnar carpal arteries, which also receive tribu taries from the descending branches of the interosseal, and the ascending of the deep palmar arch of arteries.
b. Posterior Radio-carpal Ligament.—This ligament much resembles the one last de scribed. Arising from the posterior edge of the carpal end of the radius, its fibres pass downwards and inwards to the back of the carpus, where they expand and take an ex tensive attachment to the three inner bones of the first range. The tendons of the ex tensor muscles of the fingers are related to the superficial surface of this ligament, which has also in contact with it the posterior carpal arteries from the radial and ulnar trunks.