The lower limb 577 



means of an interosseous membrane. This kind of union is a form 

 of syndesmosis (union by an interosseous ligament). The inter- 

 osseous membrane extends from the external border or interosseous 

 ridge of the tibia to the antero-internal border or interosseous ridge 

 of the fibula. The chief direction of the fibres is do^vn\vards and 

 outwards from the tibia to the fibula, but a few pass in the opposite 

 direction. Superiorly it terminates about i inch below the superior 

 tibio-fibular joint in a sharp concave margin, the concavity of 

 which is directed upwards. An interval is thus left above the 

 membrane for the passage of the anterior tibial vessels and the 

 efferent lymphatics of the anterior tibial gland. Sometimes these 

 structures pass through a distinct aperture in the membrane, called 

 the superior hiatus. Interiorly it becomes continuous with the 

 inferior interosseous hgament, and in this neighbourhood it presents 

 a small opening or inferior hiatus for the passage of the anterior 

 peroneal vessels. The interosseous membrane serves as a surface of 

 origin to muscles. 



Relations — Anterior. — Tibiahs anticus, over the upper two- thirds 

 of the inner half ; extensor longus digitorum, over the upper fourth 

 of the outer half ; extensor proprius hallucis, over the middle two- 

 fourths of the outer half ; peroneus tertius, over the lower fourth 

 of the outer half ; anterior tibial vessels, over the upper two- thirds 

 in the middle line ; and anterior tibial nerve, over the middle third. 

 Posterior. — Tibialis posticus. 



Arterial Supply. — ^The anterior tibial and peroneal arteries. 



Nerve-supply. — ^The interosseous branch of the nerve to the pop- 

 liteus muscle, which descends within it to terminate in the inferior 

 tibio-fibular joint. 



THE ARCHES OF THE FOOT. 



The foot presents two arches — antero-posterior or longitudinal 

 and transverse (see Figs. 165, 166). 



The longitudinal arch has two piers— posterior or calcaneal and 

 anterior or metatarsal. The calcaneal pier is formed by the internal 

 and external tubercles on the plantar aspect of the tuber calcis, 

 and the metatarsal pier is formed by the heads of the metatarsal 

 bones. 



The longitudinal arch consists of two pillars^intemal and 

 external. 



The internal pillar is constructed by the os calcis, astragalus, 

 navicular, three cuneiforms, and inner three metatarsal bones. Its 

 calcaneal pier is formed by the internal and external tubercles on 

 the plantar aspect of the tuber calcis, and its metatarsal pier is 

 formed by the heads of the inner three metatarsal bones. The 

 summit of the internal pillar corresponds to the upper articular 

 surface of the astragalus, and through the medium of that bone 

 the greater part of the superincumbent weight is transmitted to 

 the internal pillar. This pillar is much arched, and is character- 

 ized by great strength and elasticity. The part of it most sub- 



37 



