522 THE MUSCLES AND FASCIAE. 



brevis digitorum. They all receive a fibrous expansion from the Interossei and 

 Lumbricales, and then spread out into a broad aponeurosis, which covers the dorsal 

 surface of the first phalanx : this aponeurosis, at the articulation of the first with 

 the second phalanx, divides into three slips a middle one, Avhich is inserted into 

 the base of the second phalanx, and two lateral slips, which, after uniting on the 

 dorsal surface of the second phalanx, are continued onward, to be inserted into 

 the base of the third. 



Relations. By its anterior surface, Avith the fascia and the annular ligament ; 

 by its posterior surface, with the fibula, interosseous membrane, ankle-joint, and 

 Extensor brevis digitorum ; by its inner side, with the Tibialis anticus, Extensor 

 proprius hallucis, and anterior tibial vessels and nerve ; by its outer side, with the 

 Peroneus longus and brevis. 



The Peroneus tertius is a part of the Extensor longus digitorum, and might 

 be described as its fifth tendon. The fibres belonging to this tendon arise from 

 the lower fourth of the anterior surface of the fibula, from the lower part of the 

 interosseous membrane, and from an intermuscular septum between it and the 

 Peroneus brevis. The tendon, after passing through the same canal in the 

 annular ligament as the Extensor longus digitorum, is inserted into the dorsal 

 surface of the base of the metatarsal bone of the little toe, on its inner side. This 

 muscle is sometimes wanting. 



Nerves. These muscles are supplied by the anterior tibial nerve. 



Actions. The Tibialis anticus and Peroneus tertius are the flexors of the 

 tarsus upon the leg ; the former muscle, from the obliquity in the direction of its 

 tendon, raises the inner border of the foot ; and the latter, acting with the Pero- 

 neus brevis and longus, draws the outer border of the foot upward and the sole 

 outward. The Extensor longus digitorum and Extensor proprius hallucis extend 

 the phalanges of the toes, the action being the same as that of the corresponding 

 muscles of the hand, and flex the tarsus. Taking their fixed point from below in 

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

 dicular position. 



Posterior Tibio-fibular Region. 



Dissection (Fig. 328). 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 head, the larger and a little 

 the more posterior, arises from a depression at the upper and back part of the 

 inner condyle. The outer head arises from the upper and back part of the 

 external condyle, immediately above the origin of the Popliteus. 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, that covering the inner head being longer and thicker than 

 the outer. From the anterior surface of these tendinous expansions muscular 

 fibres are given off. The fibres in the median line, which correspond to the 



