THE SACRAL PLEXUS 175 



In addition to acting on the knee-joint, the three preceding muscles act 

 as extensors of the hip-joint, to the posterior aspect of which they are 

 related. It is interesting to observe that, while the thigh may be extended 

 without flexing the knee, the latter joint cannot be flexed unless the hip- 

 joint also is flexed at the same time. 



When the hamstrings are completely paralysed, flexion of 

 the knee is still possible but the movement is not a powerful 

 one. It is carried out by the sartorius and the gracilis, 

 which, unlike the other flexor muscles, depend for their 

 nerve-supply on the femoral and the obturator nerves 

 respectively. 



The Tibial Nerve (Internal Popliteal) arises from the 

 sciatic in the middle of the thigh and descends vertically 

 through the popliteal fossa in the middle line of the limb. In 

 the distal part of the fossa, it passes under cover of the 

 superficial muscles of the calf and runs distally to a point 

 mid-way between the medial malleolus and the point of the 

 heel, where it divides into the lateral and medial plantar 

 nerves. 



In the popliteal fossa, the tibial nerve gives off (a) Articular 

 branches to the knee-joint, (b) motor branches^ and (c) the medial 

 sural nerve (ramus communicans tibialis). 



(b) The motor branches are supplied to both heads of the 

 gastrocnemius, the plantaris, the popliteus and the soleus. 



The Gastrocnemius arises by two heads, one from the neighbourhood 

 of each femoral condyle, and these two fleshy bellies are mainly responsible 

 for the formation of the prominence of the calf. About the middle of the 

 back of the leg, they are attached to a common tendon, which is joined 

 at a more distal level by the tendon of the soleus. In this way the tendo 

 calcaneus ( Achillis) is formed and it is inserted into the posterior aspect of 

 the calcaneus. When the gastrocnemius contracts, it flexes the knee- 

 joint and plantar-flexes the ankle. 



The Soleus, which lies under cover of the gastrocnemius and projects 

 beyond its medial border, arises from the posterior aspects of both the tibia 

 and the fibula and unites with the tendon of the gastrocnemius. It acts 

 solely as a plantar flexor of the foot. 



The Plantaris is a small muscle which arises from the femur, just 

 proximal to the lateral condyle, and is inserted into the calcaneus. It has 



