182 ARTICULATIONS OF THE LOWER LIMB. 



ligament or fibrous plate ; and each of the cavities is lined by a synovial 

 membrane. 



Movements. — In the mechanism of the foot three arches are distinguishable, 

 two of them longitudinal and one transverse ; all of them cai3able of being- 

 flattened somewhat by pressui'e from above, thus securing elasticity. The inner 

 arch is formed by the os calcis, astragalus, scaphoid, and three cuneiform bones, 

 together with the three inner toes, the head of the astragalus being the key- 

 stone, and is supported by the inferior calcaneo-scaphoid ligament. The outer 

 arch is formed by the os calcis. cuboid bone, and two outer toes, and is supported 

 by the strong mferior calcaneo-cuboid ligaments. Thus the calcaneo-scaphoid 

 and calcaneo-cuboid ligaments are stretched by the whole weight of the body 

 bearing down upon the arch, and prevent the too great flattening of the instep ; 

 an action in which they are assisted, however, by the iilantar aponeurosis. The 

 transverse arching of the foot is most marked in the line of the tarso-metatarsal 

 articulations, and is maintained by the wedge-shape of the bones and by the 

 plantar ligaments. The weight of the body, fallmg upon the balls of the toes 

 when the heel is raised, tends to sjjread out the metatarsal bones at their distal 

 extremities, and to flatten the transverse arch, Avhich recovers its position when 

 the pressure is removed. Between the astragalus and the calcaneum only one 

 kind of motion is possible, the centre of which is the interosseous astragalo- 

 calcaneal ligament, and is of such a nature, that when the posterior part of the 

 OS calcis slides inwards and upwards beneath the astragalus, its cuboid extremity 

 moves downwards and outwards. A certain amount of gliding movement is also 

 allowed between the tarsal and metatarsal bones, and that mo.st considerably 

 between the cuboid bone and outer toes. Thus it happens that if the foot i.s 

 bent up against the leg, and then is gi'adually extended by force applied to the 

 toes, as by the action of the flexores digitorum muscles, the fii'st part of the 

 movement is accomplished at the ankle, and consists of extension, with only a 

 slight inward turning of the toes ; further extension is accomplished by move- 

 ment between the calcaneum and astragalus, and is accompanied by depression 

 of the outer edge of the foot ; and after that a little more extension, accom- 

 panied by more considerable depression of tlie outer edge of the foot, is effected 

 at the tarso-metatarsal articulations. The dnection of the movement of exten- 

 sion of the toes at the metatarso-phalangeal articulations is upwards and out- 

 wards, so that although the great toe is in a line with the inner edge of the 

 foot when resting on the ground, it is no longer so when over-extended. 



