152 THE ARTICULATIONS. 



fibres more or less oblique, and arranged crosswise; by its amplitude it 

 can adapt itself to the movements of flexion of the knee. 



d. The posterior ligament, one of the strongest in the animal economy, 

 covers the posterior face of the carpus, filling up the asperities which 

 roughen it. It is inserted : above, on the transverse crest surmounting the 

 articular surface of the radius ; by its middle portion into all the carpal bones ; 

 below, into the head of the principal metacarpal bone. Confounded inwardly 

 with the internal lateral ligament, united outwardly to the band which attaches 

 the supercarpal to the external metacarpal and the second carpal bone of the 

 upper row, this ligament is continued, by its inferior extremity, with the 

 carpal stay (or check ligament) which sustains the perforans tendon. Its 

 posterior face is perfectly smooth, and is covered by the synovial membrane 

 of the carpal sheath. 



MOVEMENTS OF THE CARPAL ARTICULATIONS. The carpus is the seat of 

 two very extensive and opposite movements -flexion and extension ; to which 

 are added three very limited accessory movements adduction, abduction, and 

 circumduction. 



All the carpal articulations do not take an equal part in the execu- 

 tion of these movements , for it is easy to discover that they are chiefly 

 performed in the radio-carpal diarthrosis, and in the imperfect hinge 

 articulation uniting the two rows of carpal bones. Each of these articula- 

 tions participates in the movements of the carpus in nearly the same pro- 

 portions, and both act in an identical manner. Their mechanism is most 

 simple. 



In flexion, the first tier of bones rolls backwards on the radius, the inferior 

 row moves in the same sense on the upper, the metacarpus is carried back- 

 wards and upwards, the common posterior ligament is relaxed, the capsular 

 ligament becomes tense, and the articular surfaces, particularly those of the 

 second joint, separate from each other in front. In extension, the metacarpus 

 is carried downwards and forwards by an inverse mechanism. This move- 

 ment stops when the ray of the fore-arm and that of the metacarpus are in 

 the same vertical line. In flexion, these rays never directly approach each 

 other ; the inferior extremity of the metacarpus being always carried outwards. 

 It may also be remarked, that the slight movements of abduction, adduction, 

 and circumduction of the carpus are only possible at the moment when the 

 foot is flexed on the fore-arm, 



With regard to the planiform diarthrosis articulating the carpal bones of 

 the same row, they only allow a simple gliding between the surfaces in 

 contact ; and with the carpo-metacarpal arthrodia it is absolutely the same. 

 The restricted mobility of these various articulations has but a very secondary 

 influence on the general movements of the carpus ; but it nevertheless favours 

 them by permitting the carpal bones to change their reciprocal relations, 

 and adapt themselves, during the play of the radio-carpal' and intercarpal 

 hinges, to a more exact coaptation of the articular planes which they form. 



In the other animals, the carpal articulations have the same essential characteristics 

 we have noticed in Solipeds. The four principal peripheral bands differ but little in 

 them ; though in the Dog and Cat they are lax enough to allow somewhat extensive 

 lateral movements. 



5. Intermetacarpal Articulations. 



Each lateral metacarpal bone articulates with the middle one by means 

 of diarthrodial and synarthrodial surfaces, for the description of which refer 

 to page 82. An interosseous ligament, composed of very short and strong 



