SOLEUS 373 



upon its anterior asi)oct and sides it receives fleshy fibres nearly as far down as the 

 hack of the ankle-joint. The tendo Achillis is a strong rovnided l)and of tendon 

 about three-quarters of an inch (2 cm.) from side to side, and about half as much 

 from l)efore backwards; it is narrowest at the level of the ankle-joint, and expands 

 slightly before it is inserted into the lower part of the pcjsterior surface of the cal- 

 caneum. A bursa intervenes between the tendon and the smooth upper part of tlie 

 ])Osterior surface of this bone. 



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

 by sural l)ranches which enter the upper half of the muscle upon the posterior 

 surface; and lower down by a l:)ranch from the posterior tibial nerve, which enters 

 the anterior surface of the muscle. 



Action. — The chief action of the combined gastrocnemius and soleus is to 

 extend the ankle-joint. It is an extremel}' powerful muscle, as it acts with con- 

 sideral)le mechanical disadvantage. The lever by means of Avhich it acts may be 

 best descril)ed as one of the first order (see page 285); the lever being that part of 

 the foot which lies between the heel and the heads of the metatarsal bones, the 

 ankle being the fulcrum; a pressure ecjual to the weight of the body being exertt^d 

 by the ground at the anterior extremity of the lever; and the arm, at the end of 

 which the muscle acts, being the comparatively short distance between the back of 

 the heel and the centre of the ankle-joint. When the ankle-j(jint has been com- 

 pletely extended, this muscle wall tend to adduct slightly the foot, and to invert 

 the sole, this movement being carried out in the joint between the astragalus and 

 calcaneum. Besides extending the ankle, the gastrocnemius will assist in flexing 

 the knee-joint. The chief oliject, however, which aj^pears to be gained by the 

 femoral attachment of this muscle is the addition to the rapidity of extension of 

 the foot. Like some of the other long muscles wdiich pass over two joints, the 

 gastrocnemius, if it were an inextensible ligament, would cause extension of the 

 ankle as soon as the knee was straightened by means of the great quadriceps 

 muscle. Seeing, however, that during the contraction of the quadriceps the gas- 

 trocnemius is at the same time acting, it follows that the ra])idity and amount of 

 the extension of the ankle-joint is almost doubled. By these means we obtain that 

 ra])id and powerful contraction Avhich gives the spring to the body in lea]^)ing and 

 running. We may see also how enormously strong the tendo Achillis must be, 

 as it has not only to bear the contraction of the gastrocnemius and soleus, but the 

 additional strain thrown upon it ])y the simultaneous action of the quadriceps 

 extensor of the knee. The soleus will assist in the extension of the ankle, and will 

 even be able to perform this movement somewhat feebly by itself, when in extreme 

 flexion of the knee-joint the gastrocnemius is so relaxed as to be almost powerless. 

 It is much stronger than the gastrocnemius, as may ])e inferred from the enormous 

 nunil)er of short fil)res of which it is formed. At the same time, however, as it 

 only passes over the ankle and calcaneo-astragaloid joints, the range of its move- 

 ments is very short. 



Relations. — The soleus is in relation behind with the gastrocnemius and plan- 

 taris; in front, with the flexor longus cligitorum, flexor longus hallucis, and tibialis 

 posticus, and with the posterior til:)ial vessels and nerve. 



Variations. — A second soleus is sometimes found beneath the normal muscle, and more or less 

 separate from it. It is usually inserted into the calcaneum or internal annular ligament. 



Third Layer 



The third layer is separated from the superficial layers by an a]")oneurosis called 

 the deep tibial fascia. This is attached to the inner border of the tibia internally, 

 and externally to the outer l)order of the flexor surface of the fibula, and the 

 posterior of the two external intermuscular septa. It is thin above, but below it is 

 strengthened by transverse fibres, and becomes much thicker. At the ankle it 

 blends with the deep fascia of the leg and the external and internal annular 

 ligaments. The third layer consists of two muscles — the flexor longus digitorum 

 and the flexor loncus hallucis. 



