INTERMETACARPAL JOINTS 273 



owing to the close connection of the bases of the metacarpal bones, the radial and ulnar flexors 

 and extensors of the carpus act on all by their pull on the particular bone into which they are 

 inserted. 



Abduction, or movement toward the radial side, is prevented by the impaction of the second 

 bone against the greater multangular; a little adduction is permitted, and is favoured by the 

 slope given to the hamate and fifth metacarpal bones. 



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

 present toward the palm is deepened to form the 'cup of Diogenes.' 



Muscles which act upon the four medial carpo-metacarpal joints are the flexors and ex- 

 tensors of the ^Tist and fingers, except the flexor carpi ulnaris. 



(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 

 greater multangular. 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 



Articular capsule. 



The articular capsule (figs. 300 and 301) consists of fibres which pass from 

 the margin of the articular facet on the greater multangular, 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 medial fibres are stronger than the lateral. 



The synovial membrane is lax, and distinct from the other synovial membranes of the 

 carpus. 



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

 radial, the first volar metacarpal, and the dorsalis pollicis. 



The nerves are supplied by the branches of the median to the thumb. 



Relations. — Behind are the long and short extensor tendons of the thumb, and behind 

 and laterally the tendon of the abductor pollicis longus (extensor ossis metacarpi polhcis). 

 The tendon of the flexor poUicis longus is in front and fibres of the flexor polhcis brevis and op- 

 ponens pollicis muscles are also anterior relations. To the medial side is the radial artery as 

 it passes forward into the palm of the hand. 



The movements of this joint are regulated by the shape of the articular surfaces, rather 

 than by the ligaments, and consist of flexion, extension, abduction, adduction, and circum- 

 duction, but not rotation. In flexion and extension the metacarpal bone slides to and fro upon 

 the multangular; in abduction and adduction it slides from side to side or, more correctly, re- 

 volves upon the antero-posterior axis of the joint. The power of opposing the thumb to any 

 of the fingers is due to the forward and medial obhquity of its flexion movement, which is by 

 far its most extensive motion. Abduction is very free, while arlduction is limited on account 

 of the proximity of the second metacarpal bone. The movement of the greater multangular 

 upon the rest of the carpus somewhat increases the range of all the movements of the thumb. 



Muscles which act upon the carpo-metacarpal joint of the thumb.— FZe.Tors.^Flexqr 

 pollicis brevis, flexor pollicis longus, opponens polhcis. Extensors. — Extensores poUicis brevis 

 and longus and abductor polhcis longus. A&dwctors.— Abductorcs poUicis longus and brevis. 

 Adductors. — The transverse and obhque adductor pollicis, opponens, first dorsal interosseous. 

 Muscles producing opposition. — Opponens, flexor brevis, oblique adductor. 



9. THE INTERMETACARPAL ARTICULATIONS 



Class. — Diarthrosis. Subdivision. — Arthrodia. 



The metacarpal of the thumb is not connected with any other metacarpal 

 bone. The second, third, fourth, and fifth metacarpal bones are in actual 

 contact at their bases, and are held firmly together by the follo^^dng ligaments 

 (in addition to the articular capsule) : — 



Dorsal. Volar. 



Interosseous ligaments. 



The dorsal ligaments (fig. 302) are layers of variable thickness of strong, short fibres, which 

 pass transversely from bone to bone, filling up the irregularities on the dorsal surfaces. 



The volar ligaments are transverse layers of hgamentous tissue passingfrom bone to bone; 

 they cannot be well differentiated from the other hgaments and fibrous tissue covering the bones. 



The interosseous ligaments (fig. 304) pass between the apposed surfaces of the bones, and 

 are attached to the distal sides of the articular facets, so as to close in the synovial cavities on 



18 



