876 THE MUSCLES 



Nerve-supply. — From the posterior til)i:il nerve l\v branches which enter the 

 muscle in tlie upper part of its posterior surface near its inner l)or(ler. 



Action. — This muscle, which is nmch more powerful than the flexor longus 

 digitorum, is a strong liexor of the last phalanx of the great toe, and is of great 

 importance in walking, as it presses the great toe firmly against the ground. The 

 ungual i^halanx of the great toe is the last i)art of the foot to leave the ground when 

 the step is completed; and mitil this is the case the flexor longus hallucis is strongly 

 contractetl. It Avill also help to flex the first phalanx of the great toe upon its 

 metatarsal ])one and it will act U])on the joints which intervene between the first 

 metatarsal bone and the astragalus so as to support, the arch of the instep; and, 

 finally, it will assist in the extension of the ankle-joint. By the slip which it gives 

 to the flexor longus digitorum it will help that muscle in flexing the adjacent toes. 



Relations. — Superficially, in the leg, it is covered by the soleus, and in the foot 

 by the abductor hallucis, the flexor longus digitorum, the external plantar vessels 

 and nerve; on its outer side are the peronei; on its deep aspect in the leg lie the 

 tibialis posticus and the peroneal vessels; and, after passing over the back of the 

 ankle and other joints, it lies upon the inner head of the flexor brevis hallucis. 



Variations. — An accessory portion of the muscle may be inserted into the sustentaculum tali 

 or tlie inner surfiice of the calcaneum. The slip to the flexor longus digitorum tendon may vary 

 in the number of toes to which it is distributed. 



Fourth Layer 



The fourth layer consists of one muscle— the til)ialis posticus. 



TIBIALIS POSTICUS 



The tibialis posticus — named from its position in the back part of the leg and 

 its origin from the tibia — is a thick fusiform sheet. 



Origin. — (1) The whole of the back of the interosseous ligament with the 

 exception of the lowest portion; (2) the posterior surface of the tibia close to 

 the interosseous line, from the upper end of the oblique line to the junction of the 

 middle and lower thirds of the shaft; (3) the internal (or inner part of the 

 flexor) surface of the fibula at the back of the interosseous ridge to within a short 

 distance of the ankle; (4) the intermuscular sei)ta which intervene lietAveen it and 

 the muscles of the third layer, viz. the flexor longus hallucis and the flexor longus 

 digitorum; and (o) a small portion of the deep tibial fascia which separates the 

 second from the third layer. 



Insertion. — (1) The tuberosity of the scaphoid bone; (2) by several smaller 

 offsets into the front of the lower surface of the sustentaculum tali and the under 

 surface of all the other tarsal bones with the exception of the astragalus; and (3) 

 the under surface of the bases of the second, third, and fourth metatarsal bones. 



Structure. — A strong bipenniform muscle, the central tendon of which, -lying 

 upon the middle of the l)ack of the muscle, l)egins about the middle of the leg, and 

 passes downwards and inwards upon the liack of the mviscle, receiving its' last 

 fleshy fibres aliout an inch (2-5 cm.) above the ankle. Having passed inwards 

 beneath the tendon of the flexor longus digitorum, it enters the innermost groove 

 on the back of the internal malleolus, and is contained in a synovial sheath which 

 accompanies it to its insertion upon the scaphoid bone. From this insertion strong 

 fibrous l)ands radiate backwards, outwards, and forwards to the tarsal and meta- 

 tarsal bones, being intimately blended with the ligaments by which these bones are 

 held together. This muscle, which is very strong, is contained, so to s]>eak, in a 

 h)ng four-sided case formed in the front by the interosseous membrane; at the 

 sides, by the op]>osing surfaces of the tibia and filjula; and behind, at a distance of 

 nearly half an inch (1-2 cm.) from the interosseous membrane. 1)V the intermus- 

 cular se])ta which sei)arate the muscle from tlie flexor longus hallucis and the flexor 

 longus digitorum. 



Just above its insertion into the tuberosity of the scaphoid bone, the tendon 

 often contains a sesamoid bone. 



