THE SACRAL PLEXUS 177 



foot are arranged to form a transverse and an antero-posterior arch. The 

 latter is most pronounced along the medial border of the sole and its key- 

 stone is formed by the head of the talus (astragalus), which occupies the 

 interval between the sustentaculum tali of the calcaneus (os calcis) and the 

 tuberosity of the navicular (scaphoid). The sustentaculum tali can be felt 

 immediately below the medial malleolus, and the tuberosity of the navicular 

 forms a prominent elevation about i| inches farther forwards on the medial 

 border of the foot. The plantar calcaneo-navicular ("spring") ligament 

 extends between these two bones and supports the head of the talus. 

 Although of great strength, it would be unable to support the talus were 

 it not in turn supported by the tendons of certain muscles. 



As the tendons of the tibialis posterior, the flexor digitorum longus and 

 the flexor hallucis longus proceed to their respective insertions, they are 

 closely related to the plantar surface of the plantar calcaneo-navicular 

 ligament. Further, the tendons of the two latter muscles cross one another 

 in the sole of the foot, and the tendinous sling so formed gives additional 

 support to the arch of the foot. 



So long as the tonus of these muscles is good, the antero-posterior arch 

 of the foot remains intact, but, when the muscles are poorly developed, 

 they become fatigued if called upon to lend support for prolonged periods. 

 In this event, the muscles lose their tonus and become relaxed. As a 

 result, the head of the talus is dependent for its principal support on the 

 plantar calcaneo-navicular ligament, which soon stretches. The head of 

 the talus thus sinks downwards and the antero-posterior arch of the foot 

 collapses. 



The transverse arch of the foot is most pronounced opposite the bases of 

 the metatarsal bones and it depends for its support principally on the 

 peronaeus longus (p. 179)- 



In the condition of flat-foot, both arches disappear and the elastic spring 

 of the step is entirely lost. 



After giving off the Medial Calcanean Nerve, which arises 

 near the ankle-joint and supplies a variable area of skin over 

 the medial aspect of the heel, the tibial nerve terminates, 

 about midway between the medial malleolus and the point of 

 the heel, by dividing into the medial and lateral plantar nerves. 



The distribution of the Medial Plantar Nerve in the foot 

 corresponds fairly accurately to the distribution of the median 

 nerve in the palm of the hand. It supplies four of the small 

 muscles of the sole, but its sensory branches are of greater im- 

 portance than its motor branches. The cutaneous branches 



12 



