526 THE MUSCLES AND FASCIAE. 



membrane. It then passes obliquely forward and outward, crossing over the 

 internal lateral ligament into the sole of the foot (Fig. 334), where, crossing 

 superficially to the tendon of the Flexor longus hallucis, 1 to which it is connected 

 by a strong tendinous slip, it becomes expanded, is joined by the Flexor acces- 

 sorius, and finally divides into four tendons which are inserted into the bases 

 of the last phalanges of the four lesser toes, each tendon passing through a fissure 

 in the tendon of the Flexor brevis digitorum opposite the base of the first 

 phalanges. 



Relations. In the leg : by its superficial surface, with the posterior tibial 

 vessels and nerve, and the deep transverse fascia, which separates it from the 

 Soleus muscle ; by its deep surface, with the Tibia and Tibialis posticus. In the 

 foot it is covered by the Abductor hallucis and Flexor brevis digitorum, and 

 crosses superficial to the Flexor longus hallucis. 



The Tibialis posticus lies between the two preceding muscles, and is the most 

 deeply seated of all the muscles in the leg. It commences above by two pointed 

 processes, separated by an angular interval, through which the anterior tibial 

 vessels pass forward to the front of the leg. It arises from the whole of the 

 posterior surface of the interosseous membrane, excepting its lowest part, from 

 the posterior surface of the shaft of the tibia, external to the Flexor longus 

 digitorum, between the commencement of the oblique line above, and the middle 

 of the external border of the bone below, and from the upper two-thirds of the 

 internal surface of the fibula ; some fibres also arise from the deep transverse 

 fascia and from the intermuscular septa, separating it from the adjacent muscles 

 on each side. This muscle, in the lower fourth of the leg, passes in front of the 

 Flexor longus digitorum, and terminates in a tendon which passes through a 

 groove behind the inner malleolus with the tendon of that muscle^ but enclosed in 

 a separate sheath ; it then passes through another sheath, over the internal lateral 

 ligament into the foot, and then beneath the inferior calcaneo-navicular ligament, 

 and is inserted into the tuberosity of the navicular and internal cuneiform bones. 

 The tendon of this muscle contains a sesamoid fibro-cartilage as it passes over the 

 navicular bone, and gives off fibrous expansions, one of which passes backward to 

 the sustentaculum tali of the os calcis, others outward to the middle and external 

 cuneiform and cuboid, and some forward to the bases of the second, third, and 

 fourth metatarsal bones (Fig. 335). 



Relations. By its superficial surface, with the Soleus, from which it is 

 separated by the deep transverse fascia, the Flexor longus digitorum, the posterior 

 tibial vessels and nerve, and the peroneal vessels ; by its deep surface, with the 

 interosseous ligament, the tibia, fibula, and ankle-joint. 



Nerves. The Popliteus is supplied by the internal popliteal nerve, the 

 remaining muscles of this group by the posterior tibial nerve. 



Actions. The Popliteus assists in flexing the leg upon the thigh ; when the 

 leg is flexed it will rotate the tibia inward. It is especially called into action at 

 the commencement of the act of bending the knee, inasmuch as it produces a 

 slight inward rotation of the tibia, which is essential in the early stage of this 

 movement. The Tibialis posticus is a direct extensor of the tarsus upon the leg ; 

 acting in conjunction with the Tibialis anticus, it turns the sole of the foot inward, 

 antagonizing the Peroneus longus, which turns it outward. The Flexor longus 

 digitorum and Flexor longus hallucis are the direct flexors of the phalanges, act- 

 ing as do the similar muscles of the hand, and, continuing their action, extend 

 the foot upon the leg; they assist the Gastrocnemius and Soleus in extending 

 the foot, as in the act of walking or in standing on tiptoe. 



In consequence of the oblique direction of the tendon of the long flexor the toes 

 would be drawn inward were it not for the Flexor accessorius muscle, which is 

 inserted into the outer side of its tendon and draws it to the middle line of the foot 

 during its action. Taking their fixed point from the foot, these muscles serve to 

 maintain the upright posture by steadying the tibia and fibula perpendicularly 

 1 That is, in the order of dissection of the sole of the foot. 



