280 THE ARTICULATIONS OR JOINTS. 
The synovial membrane (Fig. 217) is usually single and lines the capsule, but, 
as already explained, it has prolongations into the intermetacarpal and inter- 
“carpal series of joints. In connexion with the latter, the frequent absence of 
the interosseous ligament between the trapezium and trapezoid permits the free 
communication of this joint-cavity with that of the transverse carpal joint. 
METACARPO-PHALANGEAL JOINTS. 
In the case of the pollex this joint is constructed on the plan of a ginglymus 
diarthrosis ; the four corresponding joints of the fingers are also diarthroses of a 
slightly modified ball-and-socket variety. With the exception of the metacarpal 
bone of the pollex, each metacarpal bone has a somewhat 
spherical head articulating with a shallow oval cup upon 
the base of the first phalanx. The articulation in the 
thumb presents features similar to those of an inter- 
phalangeal joint. 
Hach joint possesses an articular capsule (Fig. 218) 
which presents very different degrees of strength in differ- 
ent aspects of the articulation. Thus, on the dorsal aspect, 
it cannot be demonstrated as an independent structure, 
but the necessity for dorsal hgaments is to a large extent 
obviated by the presence of the strong flattened expansions 
Metacarpal_ 
bone ~~ 
Accessory pal- 
mar ligament of the extensor tendons. 
The internal and external lateral ligaments (ligamenta 
collateralia, Fig. 218) are strong cord-lke bands which pass 
anak from the tubercles and adjacent depressions on the sides 
of the heads of the metacarpal bones to the contiguous 
non-articular areas on the bases of the proximal phalanges. 
They are intimately connected on their anterior aspects 
with the palmar ligaments. 
The palmar livamonts consist of thick plates of fibro- 
cartilage loosely connected to the metacarpal bones, but 
firmly adherent to the phalanges. They are placed 
between the lateral ligaments, to both of which they are 
in each case connected. Each plate is grooved on the 
palmar surface for the long flexor tendons, whilst on its 
reverse or joint surface it supports and glides upon the 
head of the metacarpal bone during flexion and extension 
of the joint. In the case of the thumb this plate of 
fibro-cartilage usually develops two sesamoid bones, and 
Lateral ligament in the case of the index finger one such sesamoid nodule 
is frequently found at the radial side of the plate. 
Fra. 218.—Meracarpo-PHALAN- An important accessory ligament is found in con- 
Sree eg INTERPHALANGEAL nexion with the four inner metacarpo - phalangeal 
Ge: articulations, viz. :— 
The Transverse Metacarpal Ligament.—This structure binds together the distal 
extremities of the four inner metacarpal bones. The name is applied to three sets 
of transverse fibres of great strength which are situated in front of the three inner 
interosseous spaces. These fibres are continuous with the palmar metacarpo- 
phalangeal ligaments at their lateral margins. 
A synovial membrane lines the investing capsule of each joint. 
ligament 
Lateral ligament 
Phalanx I.~ 
al ligamen 
Lateral ligament 
Capsule - 
INTERPHALANGEAL JOINTS. 
Of these joints there are two for each finger and one for the thumb. They all 
correspond, in being ginglymus diarthroses in “which the trochlear character of their 
articular surfaces is $ associated with one axis of movement directed transversely. 
In their general arrangement they correspond with each other, and to a large 
extent with the metacarpo- phalangeal series already described. Each is provided 
