176 THE NERVOUS SYSTEM 



a long thin tendon, which is placed between the gastrocnemius and the 

 soleus. Its action is the same as that of the gastrocnemius. 



The Popliteus arises from the lateral aspect of the lateral condyle, within 

 the articular capsule of the knee-joint. It crosses the lateral aspect of the 

 joint and is inserted into the proximal part of the posterior surface of the 

 tibia. When it contracts, the popliteus flexes the knee and rotates the leg 

 medially on the femur. This muscle is deeply placed, in the floor of the 

 distal part of the popliteal fossa, and its contractions cannot be appreciated 

 on the surface of the limb. 



The Medial Sural Nerve descends in the interval between 

 the two heads of the gastrocnemius and pierces the deep 

 fascia. It unites with the ramus anastomoticus peronseus 

 (nervus communicans fibularis) (p. 178) to form the nervus 

 suralis (short saphenous), and this nerve supplies the skin on 

 the postero-lateral aspect of the leg, the lateral border of the 

 foot and of the little toe. 



As the tibial nerve descends through the posterior com- 

 partment of the leg, it gives off no sensory branches but it 

 supplies the group of deep muscles, which includes the tibialis 

 posterior, the flexor digitorum and hallucis longus. 



The Tibialis Posterior arises from the posterior aspects of both bones of 

 the leg and descends behind the medial malleolus. It then passes forwards 

 and is inserted mainly into the tuberosity of the navicular bone, but it gives 

 off additional slips to all the other tarsal bones except the talus and, also, 

 to the middle three metatarsals. When it contracts, the tibialis posterior 

 plantar- flexes the foot and, at the same time, it inverts the foot, i.e. lifts 

 the medial border of the foot from tfie ground so that the sole of the foot 

 looks medially. But, while these movements are the result of the active 

 contraction of the muscle, its normal tonus helps to maintain the arches of 

 the foot. 



The Flexor Digitorum Longus and the Flexor Hallucis Longus arise, 

 respectively, from the posterior aspects of the tibia and of the fibula. They 

 pass behind the medial malleolus and then run forwards in the sole of the 

 foot. Their actions on the digits are clearly explained by their names, 

 but, in addition, they both assist the movement of plantar flexion at the 

 ankle. Further, like the tibialis posterior, they both help to maintain the 

 normal arches of the foot. 



The Arches of the Foot. In order to distribute the weight of the body 

 and to give elasticity to the step in walking and running, the bones of the 



