Besides the palmar and dorsal ligaments already described, these mctarcarpal bones are very firmly connected to each other by short but strong ligaments, extending transversely from one to the other on the palmar and dorsal aspects.
A common synovial membrane extends throughout the four joints above described ; this synovial membrane is continuous with that between the two rows of carpal bones.
The digital extremities of the four inner metacarpal bones are connected by their trans verse ligaments situated at the palmar surface and extending from one to the other.
Carpamietacarpal joints if the thumb.—The main feature by which this articulation is dis tinguished from the other carpo-metacarpal joints is its great mobility. It is an arthrodia, and in many particulars resembles very much the sterno-clavicular joint. The trapezium presents a surface concave from within out wards, and convex from before backwards, that on the metacarpal bone being convex in the transverse, and concave in the antero-pos terior direction.
The ligamentous apparatus of this joint has very much the appearance of the capsular ligament of an enarthrosis, and bas indeed been described as such by many anatomists ; but on a careful examination it will be found to consist of separate bundles of ligament placed at those situations in which the greatest tendency to displacement exists in the various motions of the joint. Four principal bundles may be described : one very thick and strong, situated at the posterior and outer part of the joint, ( lig. dorsale, \Veitbr.) extending from the metacarpal bone to a prominent tubercle on the outer part of the dorsal surface of the trapezium; this ligament limits flexion of the joint. A second ligament is situated directly in front of the joint, ( lig. palmare, \Veitbr.) is inserted into the trapezium immediately internal to its prominence; extension is limited by this ligament. The third and fourth bundles ( lig. laterale ext. et int. Weitbr.) are situ ated on the radial and ulnar sides of the joint : they are less distinct as well as less strong than those last described. That on the ulnar side
is considerably the stronger; it limits abduc tion of the thumb, whilst that on the radial side limits adduction.
The synovial membrane of this joint is lax ; it is perfectly distinct from the general syno vial membrane of the other carpo-metacarpal articulations.
Motions cif the carpo-metacarpal joints.— In the second, third, and fourth joints the motions are limited to a very slight, and during life scarcely appreciable gliding forwards or backwards: the strong transverse ligaments, as well as the close manner in which the carpal extremities of the metacarpal bones are im pacted together, render lateral motion impos sible; in the fifth joint the forward or back ward motion is somewhat more extensive, but this joint is equally limited with the others in lateral movement.
The carpo-metacarpal joint of the thumb enjoys motion forwards, backwards, inwards, and outwards, producing the movements of flexion and extension, abduction and ad duction. The power of opposing the thumb to any of the fingers is due to the oblique direction of flexion in this joint : the bone moves forwards and inwards, passing through a line which would be concave inwards. This is by far the most extensive motion of the thumb, and it is by an excess of this motion that the dislocation of the metacarpal bone backwards is generally occasioned. Cruveil hier observes that the weakness of the posterior ligament favours the occurrence of this lux ation. I cannot, however, admit the weakness of this ligament ; on the contrary, it appears to me to be the strongest of all the ligaments of this joint ; which opinion, I find, is that of the accurate NVeitbrecht.
The motion of adduction is, on the other hand, the most limited, in consequence of the proximity of the second metacarpal bone; that of abduction is very extensive, and when car ried too far may occasion luxation inwards.