520 A MANUAL OF ANATOMY 



simply an upward expansion of the tendon of insertion of the 

 muscle. 



Tendo Achillis.* — This very strong tendon is formed by the 

 union of the tendons of the gastrocnemius and soleus. It extends 

 from the centre of the leg to the prominence of the heel, and is 

 inserted into the middle zone on the posterior surface of the 

 tuber calcis, a bursa intervening between it and the upper zone. 

 The tendo Achillis is at first broad and comparatively thin. As 

 it descends, it becomes narrower and thicker until it reaches 

 a point about i| inches above the tuber calcis, beyond which it 

 again broadens to its insertion. The short saphenous vein and 

 nerve lie on its outer side, the tendon of the plantaris and the 

 posterior tibial vessels and nerve on the inner side, and a large 

 quantity of fat beneath it. 



Plantaris — Origin. — (i) The lower 2 inches of the external 

 supracondylar ridge of the femur, internal to the external head of 

 the gastrocnemius ; and (2) the adjacent part of the posterior 

 ligament of the knee-joint. 



Insertion. — The middle zone on the posterior surface of the 

 tuber calcis immediately to the inner side of the tendo Achillis, 

 with which it is closely connected. It may, however, terminate 

 upon the inner border of the tendo Achillis, in the deep fascia of 

 the leg, upon the internal annular ligament, or in the plantar 

 fascia. 



Nerve-supply. — ^The internal popliteal nerve. 



Action. — (i) The plantaris acts as a very weak auxiliary to the 

 gastrocnemius. (2) From its connection with the posterior liga- 

 ment of the knee-joint the muscle has a slight action as a retractor 

 of that ligament during flexion of the joint, and thus it guards the 

 ligament against being nipped or pressed upon by the articular 

 surfaces of the femur and tibia. 



The fleshy belly of the muscle is limited to the upper sixth. 

 The tendon is very narrow and descends inwards between the 

 gastrocnemius and soleus. It is also very extensible, so that when 

 grasped by the fingers and stretched laterally it can be drawn out 

 into a ribbon of about 2 inches in breadth. 



The plantaris, which is sometimes absent, is a vestigial muscle, being the 

 crural remains of a superficial flexor of the toes, the plantar portion of the 

 tendon of which, having been divorced, remains persistent as the central f 

 division of the plantar fascia. The homologue of the plantaris in the upper 

 limb is the palmaris longus. 



Deep Group. — The muscles which comprise this group are four ■:; 

 in number, namely, the popliteus, flexor longus digitorum, tibialis [i 

 posticus, and flexor longus hallucis. The popliteus muscle is i; 

 covered by the popliteal fascia, which is one of the modes of inser- j; 

 tion of the semimembranosus. The other muscles, together with | 



* Being inserted into the heel, it has been called the tendo Achillis, becausie 

 it was believed that the heel was the only part in which the Greek hero, Achilles, j 

 Qould be wounded. ' | 



