372 THE MUSCLES 



part of the outer surface of the external condyle of the femur; also by a small slip 

 from the posterior ligament of the knee-joint. 



Insertion. — (1) The back of the tibia from below the attachment of the pos- 

 terior ligament of the knee-joint to the oblique line; (2) the fascia derived from 

 the tendon of the semi-memljranosus, which covers the posterior surface of the 

 muscle. 



Structure. — Arising by a somewhat flattened cylindrical tendon which passes 

 at tirst backwards and slightly downwards within the knee-joint, grooving the 

 posterior border of the external semilunar cartilage; it then escapes from tlie capsule 

 of the knee-joint, receiving a small slip from the posterior ligament, and innne- 

 diately expands into a fan-shaped muscle which forms a thick sheet, covering the 

 upper fourth of the back of the tibia, and is inserted by fleshy fibres into the pouch 

 formed by the bone in front and the aponeurosis derived from the semi-membra- 

 nosus behind. The tendon of origin is surrounded by synovial membrane, which 

 is reflected upon it aljout half an inch (1'2 cm.) beyond the opening, through 

 which it emerges from the posterior ligament of the knee-joint. 



Nerve-supply. — From the sacral plexus by the internal popliteal division of the 

 great sciatic nerve, which sends a special branch round the lower border of the 

 muscle to distribute itself to the lower part of its deep or anterior surface. 



Action. — To flex the knee, which it will do but feebly on account of the 

 obliquity of its direction and its proximity to the axis of the joint. When the knee 

 is flexed it will act as an internal rotator of the leg. In this position the tendon of 

 origin lies wholly in the groove for its reception upon the outer surface of the 

 external condyle. It is possible that the attachment of the tendon of origin to the 

 posterior ligament of the knee may enable the muscle when it contracts to draw 

 backwards the ligament, and so prevent the synovial membrane upon the anterior 

 aspect of the ligament from being nipped lietween the articular surfaces. 



Relations. — Behind, the aponeurosis of the semi-meml)ranosus, the gastroc- 

 nemius, plantaris, and the popliteal vessels, the internal pojiliteal nerve, and 

 the lymphatic glands. In front, the knee-joint. Superficial to the tendon of origin 

 is the external lateral ligament of the knee. 



Variations. — A second head of origin has been seen from a sesamoid bone in the outer tendon 

 of the gastrocnemius. 



2. SOLEUS 



The soleus — named from solea^ the Latin for a sole-fish, because of the resem- 

 blance of the muscle to this flat fish — is a thick, fusiform sheet which forms the 

 lowest head of the triceps surse muscle. 



Origin. — (1) The oblicjue line of the tibia and the inner border of its posterior 

 surface, from the lower end of the oblique line to a little below the middle of the 

 leg; (2) the back of the head and the upper third of the outer border of the 

 posterior or flexor surface of the filjula and the adjacent external intermuscular 

 septum; (3) a tendinous arch Avhich stretches across the interval between the 

 upjH'r part of the back of the tibia and fibula. 



Insertion. — By a strong aponeurosis which l)lends with the anterior surface of 

 the tendon of the gastrocnemius, and forms the tendo Achillis. 



Structure. — The muscle arises partly by fleshy fibres and partly b}^ a strong 

 aponeurosis, which lies in front of the fleshy fibres, and is especially noticeable in 

 the neighl)Ourhood of its tibial attachment. From the til)ia and fibula the fibres 

 pass in a bipenniform arrangement downwards and towards the middle line, and 

 after a very short course, not exceeding two inches (5 cm. ) in length, they l)lend 

 with the tendon of insertion which begins near the upper part of the origin of the 

 muscle, and in cross-sections of the muscle resembles in shape the letter T; one 

 part of it forming a broad aponeurosis upon the })osterior surface of the muscle, 

 the other part a strong tendinous sejjtum which passes forwards from the middle of 

 the broad aponeurosis a1)ove mentioned, so as to separate the fleshy fibres into two 

 portions. About the junction of the middle and loAver thirds of the leg, the tendon 

 of insertion joins by its posterior aspect with that of the gastrocnemius muscle. l)ut 



