522 A MANUAL OF ANATOMY 



tendons which, after having given origin to the four lumbricales 

 muscles, go to the four outer toes. Each tendon, as it passes along 

 the plantar surface of a toe, has a tendon of the flexor brevis 

 digitorum lying close beneath it as far as the second phalanx, 

 the two tendons occupying a fibro-osseous canal lined by a synovial 

 membrane, which furnishes a separate investment to each tendon. 

 Opposite the first phalanx the brevis tendon divides into two parts, 

 and the longus tendon passes through the cleft thus formed (hence 

 the name flexor perforans) on its way to the distal phalanx. The 

 fibro-osseous canals and their synovial membranes, with the vincula 

 accessoria tendinum of the latter, namely, ligamenta brevia and 

 ligamenta longa, correspond with those of the fingers. 



In the leg the muscle occupies a fibro-osseous canal formed by 

 intermuscular septa and the posterior surface of the shaft of the 

 tibia. 



Tibialis Posticus — Origin. — (i) The. outer division of the posterior 

 surface of the shaft of the tibia, extending as high as the commence- 

 ment of the oblique or popliteal line, and ceasing just below the 

 centre of the bone ; (2) the posterior surface of the interosseous 

 membrane, except the lower 2 inches ; (3) the internal surface of 

 the shaft of the fibula ; (4) the posterior or transverse intermuscular 

 septum, as it covers the muscle ; and (5) the intermuscular septa 

 on either side, separating it from the flexor longus digitorum 

 internally and the flexor longus hallucis externally. 



Insertion. — The tuberosity of the navicular or scaphoid bone. 

 From this insertion eight expansions are given off. One passes 

 backwards to the anterior part of the under surface of the susten- 

 taculum tali of the os calcis. The other seven pass forwards, and 

 are attached as follows : three to the internal, middle, and external 

 cuneiform bones, one to the cuboid bone, and three to the bases of 

 the second, third, and fourth metatarsal bones, all on their plantar 

 aspects. 



Nerve-supply. — ^The posterior tibial nerve. 



Action. — (i) The muscle inverts the sole of the foot ; (2) it is an 

 extensor of the foot upon the leg ; and (3) it contributes in a material 

 degree +0 the support of the longitudinal arch of the foot, and so 

 helps to guard against the condition known as flat foot, being in 

 this respect auxiliary to the inferior or internal calcaneo-navicular 

 or spring ligament. 



In the lower third of the leg the muscle passes obliquely in- 

 wards in front of the flexor longus digitorum. The tendon passes 

 through the groove behind the internal malleolus, where it is 

 beneath the internal annular ligament, occupying one of the fibro- 

 osseous canals, and having the tendon of the flexor longus digi- 

 torum (enclosed within its fibrous canal) lying behind and slightly 

 external to it. In this part of its course the tendon has a special 

 synovial investment, which is also prolonged forwards around it 

 as far as the navicular bone. As it ])asses over the head of the 

 astragalus and spring ligament it contains a sesamoid fibrocartilage. 



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