INTERPHALANGEAL JOINTS. 351 



ARTICULATIONES DIGITORUM PEDIS. 



Interphalangeal Joints. Each toe possesses two interphalangeal joints except 

 the great toe, which has only one. Not infrequently in the little toe the distal 

 joint is obliterated through ankylosis. All the joints of this series are uniaxial or 

 hinge joints. The nature of the articular surfaces closely resembles the correspond- 

 ing joints in the fingers. 



Each joint possesses an articular capsule which is either very thin or limited to 

 the synovial stratum on the dorsal aspect. The plantar surface of the capsule is 

 strengthened by a fibrous plate. The ligamenta collateralia are well-defined bands 

 similar to those already described in connexion with the metatarso-phalangeal 

 joints. 



A synovial stratum lines each capsule in the series. The epiphyseal lines are 

 extra-capsular. 



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 calcaneus constitutes a rigid and stable pier of support, while anteriorly, by in- 

 creasing 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 talus, which 

 receives the weight of the body from the tibia, and the resilience of the arch is assured by the 

 calcaneo - navicular, calcaneo-cuboid, and long plantar ligaments, together with the plantar 

 aponeurosis, 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, a 

 medial and a lateral. The former, consisting of the talus, navicular, and the three cuneiforms, 

 with the three medial metatarsal bones, distributes weight through the talo-navicular joint, 

 while the latter (i.e. the lateral column), comprising the calcaneus, cuboid, and the two lateral 

 metatarsal bones, acts in a similar manner through the talo-calcanean 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 the 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, it 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, i.e. 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 anterior 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 muscles. 



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 medial border of the foot so that the sole looks medially, 

 while the toes are depressed towards the ground, and the lateral 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 lateral 

 border of the foot is raised from the ground, while the medial border is depressed. In both of these 

 movements there is rotation between the talus and calcaneus about an oblique axis which 

 'passes from the medial side of the neck of the talus to the lateral and inferior part of the 

 calcaneus. 



Of course, all the movements of the foot are subordinated to its primary functions as an organ 

 Df support and progression. For these purposes its longitudinal and transverse arches are of 

 extreme importance. The longitudinal arch resting on the calcaneus behind and the heads of 



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