492 THE INFERIOR EXTREMITY 



and the flexor digitorum brevis, and its terminal branches join the digital 

 arteries. 



The lateral plantar artery runs forwards and laterally towards the base of 

 the fifth metatarsal, where it bends sharply medially, and becomes the plantar 

 arch. The arch lies on the bases of the middle three metatarsals, and is 

 completed at the posterior end of the first inter-metatarsal space, where it 

 is joined by the dorsalis pedis artery. 



Gangrene of the Foot. When senile gangrene is due to 

 gradual obliteration of the small arteries of the foot, it begins 

 at the periphery and spreads in a proximal direction. The 

 pathological process tends to be delayed, and may even come 

 to a sudden stop, on the distal side of the large joints. The 

 position of these lines of demarcation is determined by the 

 anastomoses, which occur round the ankle (p. 482), the knee 

 (p. 445), and the hip (pp. 416 and 418). 



On the other hand, when gangrene is due to obstruction of 

 one of the main arterial trunks by an embolus, its onset is sudden 

 and a large area is rapidly involved. 



The Architecture of the Foot is so planned that the 

 weight of the body may be equally distributed over the bearing 

 surface. Under normal conditions the foot forms a tripod, 

 the points of which are the calcaneus and the heads of the first 

 and fifth metatarsal bones. To a much lesser extent, the heads 

 of the middle three metatarsals and the lateral border of the 

 foot help to support the body-weight. This arrangement is 

 maintained by the presence of two intersecting arches, of which 

 one is longitudinal and the other transverse. Each is supported 

 by ligaments, muscles and tendons, which render it somewhat 

 elastic. On this account the arches flatten out slightly when 

 the weight of the body is borne upon the feet and contract again 

 as soon as the weight is removed. 



The Longitudinal Arch is constituted by the medial border 

 of the foot, and its keystone is formed by the head of the talus. 

 The short posterior limb of the arch is formed by the calcaneus, 

 while the navicular, cuneiforms, and medial three metatarsals 

 constitute the longer anterior limb. The head of the talus is 

 supported by the plantar calcaneo- navicular ("spring") 

 ligament, which extends between the sustentaculum tali and 

 the navicular (Fig. 144). This ligament is supported, in its 

 turn, by the strong tendons of the tibialis posterior, the flexor 

 digitorum longus and the flexor hallucis longus, which are 

 related to its medial border and plantar aspect. The two flexor 

 tendons cross one another in the region of the plantar calcaneo- 



