PLANTARIS—POPLITEUS 371 



Nerve-supply. — From the internal j)()})liteal l)ranch of the ^reat sciatic nerve, 

 which sends sural branches to the adjacent portions of the anterior surfaces of the 

 two heads, in the upper third of the leg. 



Action. — Its action will l)e described with that of the soleus, which forms a 

 part of the same muscle. 



Relations. — Behind, the deep fascia, the external saphenous vein and nerve, 

 and the connnunicans peronei nerve. Between the two heads above is the plantaris 

 muscle. In front lie the knee-joint, the tendon of the semi-nrembranosus, the pop- 

 liteus, the i)lantaris tendon, the soleus, the popliteal vessels, and internal jxtpliteal 

 nerve. On the outer side, aljove, are the biceps tendon, and external po})lilcal 

 nerve; on the inner side, above, are the tendons of the semi-tendinosus, gracilis, 

 sartorius, and adductor magnus. A bursa lying beneath its inner head separates 

 it from the tendon of the semi-membranosus, and communicates with the knee-joint. 



Variations. — The most comnion variation is the addition of a third head from the posterior 

 surface of the lower end of the femur. This may cross, or even run between, the popliteal 

 vessels. 



2. PLAXTARIS 



The plantaris — named from its occasional attachment to the fascia covering the 

 sole of the foot ( ^plaitta) — is a fusiform, sonjewhat flattened muscle with a very 

 long ribbon-shaped tendon. 



Origin. — (1) The lower two inches (5 cm. ) of the outer condylar ridge together 

 with the posterior surface of the femur immediately below that ridge; and (2) 

 the adjacent part of the posterior ligament of the knee-joint. 



Insertion. — The inner side of the lower portion of the posterior surface of the 

 calcaneum; sometimes, however, it Ijlends with the inner l)order of the tendo 

 Achiliis, and sometimes it is continued into the inner division of the plantar fascia. 



Structure. — Arising fleshy , the fibres of this small muscle have a somewhat 

 penniform arrangement and converge upon the thin tendon, which ap})ears first on 

 the inner side of the muscle, and soon l^ecoming free, runs downwards and slightly 

 inwards across the calf between the gastrocnemius and the soleus. In the lower 

 third of the leg it lies along the inner jjorder of the tendo Achiliis, with which it is 

 sometimes l)lended. 



Nerve-supply. — From the internal popliteal Ijranch of the great sciatic nerve 

 by a small filament which enters the deep aspect of the muscle near the upper part 

 of its inner border. 



Action. — This vestigial muscle is a feeble extensor of the ankle and a flexor of 

 the knee-joint. By its attachment to the posterior ligament of the knee-joint it will 

 tend to draw liackwards that ligament during flexion of the knee, and so prevent its 

 being caught between the articular surfaces. 



Relations. — Behind lie the fascia of the popliteal space, the biceps, the gastroc- 

 nemius, and the external popliteal nerve; in front are the popliteal vessels and 

 internal popliteal nerve, the popliteus muscle and the soleus. 



Variations. — In addition to the above-mentioned variations in the point of insertion, this 

 muscle may sometimes be double at its origin, and it is not unfrequeutlj' absent. 



Second Layer 



The second layer is formed above by the popliteus, which is covered behind by 

 the aponeurosis derived from the semi-membranosus; and below l)y the soleus, 

 which is the lower head of the great triceps sura3 muscle. 



1. POPLITEUS 



The popliteus (fig. 202) — named from its position on the floor of the ham 

 (^:=: pnphs ) — is a triangular sheet. 



Origin. — The bottom of the anterior portion of a horizontal groove on the lower 



