242 THE ARTirVLATIOXS 



slitrht, as there is no Ion,? flexor attached to either; ))Ut, owing to the close connec- 

 tion of the bases of the nietacarpal l)ones, the radial and ulnar flexors and ex- 

 tensors of the carpus act on all by their i)ull on the particular l>one into whicli 

 they are inserted. 



Abduction, or movement toward the radial side, is i)revented by the impaction 

 of the second ))one against the trapezium; a little adduction is permitted, and is 

 favoured l)y the slope given to the unciform and fifth metacarpal bones. 



There is also a slight gliding between the fourth and fifth bones, when the 

 concavity they present towards the palm is deepened to form the 'cup of Diogenes.' 



(6) The Carpo-metacarpal Joint of the Thumb 

 Class. — Diarthrosis. Subdivision. — Saddle- shaped Arthrodia. 



The bones entering into this joint are the base of the first metacarpal, and the 

 trapezium. The first metacarpal bone diverges from the other four, contrasting 

 very strongly with the position of the great toe. It is due to this divergence that 

 the thumb is able to be opposed to each and all the fingers. The ligament which 

 unites the bones is the 



Capsular. 



The capsular ligament (figs. 231 and 232) consists of fibres which pass from 

 the margin of the articular facet on the trapezium, to the margin of the articular 

 facet at the base of the first metacarpal bone. The fibres are stronger on the dorsal 

 than on the palmar aspect. They are not tense enough to hold the bones in close 

 contact, so that while they restrict they do not prevent motion in any direction. 

 The internal fibres are stronger than the external. 



The synovial membrane is lax, and distinct from the other synovial mem- 

 branes of the carpus. 



The arteries of the carpo-metacarpal joint of the thumb are derived from the 

 trunk of the radial, the arteria princeps pollicis, and the dorsales pollicis. 



The nerves are supplied l)y the Ijranches of the median to the thumb. 



The movements of tliis joint are regulated by the shape of the articular sur- 

 faces, rather than l)y the ligaments, and consist of flexion, extension, abduction, 

 adduction, and circumduction, but not rotation. In flexion and extension the 

 nietacarpal bone slides to and fro upon the trapezium; in abduction and adduction 

 it slides from side to side or, more correctly, revolves upon the antero-posterior axis 

 of tlie joint. The power of opposing the thumb to any of the fingers is due to the 

 forward and iuAvard obli<iuity of its flexion movement, which is by far its most 

 extensive motion. Abduction is very free, while adduction is limited on account 

 of the proximity of the second metacarpal bone. The movement of the trapezium 

 upon the rest of the cari)us somewhat increases the range of all the movements of 

 the thumb. 



9. THE INTERMETACARPAL ARTICULATIONS 

 Class. — Diarthrosis. Subdivision. — ArtJirodia. 



Tlie metacarpal of the thumb is not connected with any other metacarpal bone. 

 The second, third, fourth, and iifth metacarpal l>ones are in actual contact at their 

 bases, and are held firmly together l)y the following ligaments : — 



Dorsal. Palmar. 



Interosseous ligaments. 



The dorsal ligaments ( fig. 233) are layers of varial)le thickness of strong, short 

 fibres, wliich ])ass transversely from bone to bone, filling up the irregularities on 

 the dorsal sm-faccs. 



The palmar ligaments are transverse layers of ligamentous tissue passing from 



