436 THE MUSCLES AND FASCIA. 



foot at the ankle-joint; the former muscle, when acting in conjunction with the 

 Tibialis posticus, raises the inner border of the foot (i. e., inverts the foot); and 

 the latter, acting with the Peroneus brevis and longus, draws the outer border of 

 the foot upward and the sole outward (i. e., everts the foot). The Extensor longus 

 digitorum and Extensor proprius hallucis extend the phalanges of the toes, and, 

 continuing their action, flex the foot upon the leg. Taking their fixed point from 

 below, in the erect posture, all these muscles serve to fix the bones of the leg in the 

 perpendicular position, and give increased strength to the ankle-joint. 



6. Posterior Tibio-fibular Region. 



Dissection (Fig. 255). Make a vertical incision along the middle line of the back of the 

 leg, from the lower part of the popliteal space to the heel, connecting it below by a transverse 

 incision extending between the two malleoli ; the flaps of integument being removed, the fascia 

 and muscles should be examined. 



The muscles in this region of the leg are subdivided into two layers super- 

 ficial and deep. The superficial layer constitutes a powerful muscular mass, 

 forming the calf of the leg. Their large size is one of the most characteristic 

 features of the muscular apparatus in man, and bears a direct connection with his 

 ordinary attitude and mode of progression. 



Superficial Layer. 

 Gastrocnemius. Soleus. Plantaris. 



The Gastrocnemius is the most superficial muscle, and forms the greater part 

 of the calf. It arises by two heads, which are connected to the condyles of the femur 

 by two strong flat tendons. The inner and larger head arises from a depression at 

 the upper and back part of the inner condyle and from the adjacent part of the 

 femur. The outer head arises from an impression on the outer side of the external 

 condyle and from the posterior surface of the femur immediately above the condyle. 

 Both heads, also, arise by a few tendinous and fleshy fibres from the ridges which 

 are continued upward from the condyles to the linea aspera. Each tendon spreads 

 out into an aponeurosis, which covers the posterior surface of that portion of the 

 muscle to which it belongs ; the muscular fibres of the inner head being thicker and 

 extending lower than those of the outer. From the anterior surface of these tendi- 

 nous expansions muscular fibres are given off. The fibres in the median line, which 

 correspond to the accessory portions of the muscle derived from the bifurcations of 

 the linea aspera, unite at an angle upon a median tendinous raphe below : the 

 remaining fibres converge to an aponeurosis which covers the anterior surface of 

 the muscle, and this, gradually contracting, unites with the tendon of the Soleus, 

 and forms with it the tendo Achillis. 



Relations. By its superfiical surface, with the fascia of the leg, which separates 

 it from the external saphenous vein and nerve ; by its deep surface, with the pos- 

 terior ligament of the knee-joint, the Popliteus, Soleus, Plantaris, popliteal vessels, 

 and internal popliteal nerve. The tendon of the inner head corresponds with the 

 back part of the inner condyle, from which it is separated by a synovial bursa, 

 which, in some cases, communicates with the cavity of the knee-joint. The ten- 

 don of the outer head contains a sesamoid fibro-cartilage (rarely osseous) where it 

 plays over the corresponding outer condyle ; and one is occasionally found in the 

 tendon of the inner head. 



The Gastrocnemius should be divided across, just below its origin, and turned downward, 

 in order to expose the next two muscles. 



The Soleus is a broad flat muscle situated immediately beneath the Gastroc- 

 nemius. It has received its name from its resemblance in shape to a sole-fish. It 

 arises by tendinous fibres from the back part of the head of the fibula and from 

 the upper third of the posterior surface of its shaft ; from the oblique line of the 



