304 THE ARTICULATIONS OR JOINTS. 
Each joint possesses a capsule (Figs. 228 and 230) which is either very thin 
or limited to synovial membrane on the dorsal aspect. The plantar surface of the 
capsule is strengthened by a fibrous plate. The lateral ligaments (ligamenta 
collateralia) are well-defined bands similar to those already described in connexion 
with the metatarso-phalangeal joints. 
A synovial membrane lines each capsule in the series. 
Mechanism of the foot.—The bones of the foot are arranged in the form of a longitudinal 
and a transverse arch. The longitudinal arch is built on a very remarkable plan. Posteriorly 
the mass of the os calcis constitutes a rigid and stable pier of support, while anteriorly, by increasing 
the number of component parts, the anterior pier acquires great flexibility and elasticity without 
sacrificing strength or stability. The summit of the arch is formed by the astragalus, which 
receives the weight of the body from the tibia, and the resilience of the arch is assured by the 
caleaneo-scaphoid and calcaneo-cuboid ligaments, together with the plantar fascia, which act as 
powerful braces or tie bands, preventing undue separation of the piers of the arch, and consequent 
flattening of the foot. The weight of the body is distributed over all the five digits, owing to ‘ 
the arrangement of the bones of the foot in two parallel columns, an inner and an outer. The — 
former, consisting of the astragalus, scaphoid, and the three cuneiforms, with the three inner _ 
metatarsal bones, distributes weight through the talo-scaphoid joint, while the latter (7.e the — 
outer column), comprising the caleaneum, cuboid, and the two outer metatarsal bones, acts im a q 
similar manner through the talo-caleanean joint. The main line of immobility of this arch 
passes from the heel forwards through the middle toe, but its anterior section, which is slender, 
is supported on either side by two metatarsal bones, with their proximal tarsal associations, in 
all of which greater freedom of movement is found. The transverse arch is most marked at the 
level of tarso-metatarsal articulations. The intersection of these two arches at right angles to 
each other introduces an architectural feature of great importance in connexion with the 
support of heavy weights. These longitudinal and transverse arches of the foot are in effect 
“vaults” intersecting each other at right angles, and in relation to the area which is common to 
both “vaults” the body weight is superposed exactly as the dome of a cathedral is carried 
upon two intersecting vaults. 
Movements at the Joints of Tarsus, Metatarsus, and Phalanges——Considered in detail, 
the amount of movement which takes place between any two of these bones is extremely small, 
and, so far as the tarsus and metatarsus are concerned, 1t is mostly of the nature of a gliding 
motion. 
At the metatarso-phalangeal and interphalangeal joints movement is much more free, and 
is of the nature of flexion (bending of the toes towards the sole of the foot, ze. plantar flexion) 
and extension. The latter movement when continued so as to raise the toes from the ground, 
and bend or approximate them towards the front of the leg, is termed dorsiflexion. Coincident 
with dorsiflexion there is always associated a certain amount of spreading of the toes, which is 
called abduction, and similarly with prolonged flexion there follows a diminution or narrowing 
of the transverse diameter of the front part of the foot by drawing the toes together—a move- 
ment termed adduction. In the foot the movements of abduction and adduction take place in 
regard to a plane which bisects the foot antero-posteriorly through the second toe, for this toe — 
carries the first and second dorsal interosseous muscle. 
Notwithstanding the small amount of possible movement in connexion with individual — 
tarsal and metatarsal joints, yet the sum total of these movements is considerable as regards the 
entire foot. In this way the movements of inversion and eversion of the foot result. By 
inversion we mean the raising of the inner border of the foot so that the sole looks inwards, 
while the toes are depressed towards the ground, and the outer border of the foot remains down- 
wards. This takes place chiefly at the talo-calcanean joint, but the transverse tarsal joints also 
participate. 
Eversion is chiefly the opposite of inversion, and the return of the foot to the normal position 
of the erect attitude ; but under certain conditions it may be carried further, so that the outer 
border of the foot is raised from the ground, while the inner border is depressed. In both of these 
movements there is rotation between the astragalus and os calcis about an oblique axis which 
passes from the inner side of the neck of the astragalus to the outer and lower part of the os 
calcis. - 4 
Of course all the movements of the foot are subordinated to its primary functions as an organ 
of support and progression. For these purposes its longitudinal and transverse arches are of 
extreme importance. The longitudinal arch resting on the os caleis behind and the heads of 
the metatarsal bones in front receives the weight of the body, as already explained, on the summit 
of the astragalus in the line of the third toe. Hence it is that the inner malleolus appears to be 
unduly prominent on the inner side of the ankle. The transverse arch buttresses the longitudinal 
one, and therefore, whether the body weight fall to the outer or the inner side of the longitudinal 
arch, it is supported by a mechanism at once stable, flexible, and elastic, or resilient, and capable 
of reducing to a minimum all jars that may be received by the forepart of the foot. As the heel 
is raised, in the act of walking, the weight is gradually transferred from the outer to the inner 
side of the foot, until the foot fmally leaves the ground with a propulsive movement, which 
results from flexion of the phalanges of the great toe. In this connection it is worthy of note 
that the longitudinal line of greatest strength is on the inner side of the longitudinal arch, 2e 
in relation to the great toe. 
