1010 THE NERVOUS SYSTEM 



In the upper part of the popliteal space the tibial nerve lies relatively superficially, being 

 covered dorsally by the skin and fascia, while in the lower part of the space it is overlapped by 

 the heads of the gastrocnemius and is crossed by the plantaris. In the upper part of the space 

 it Ues in front of the posterior femoral cutaneous (small sciatic) nerve and to the lateral side of 

 the vera and artery; at the middle of the space it is dorsal and in the lower part of the space 

 it is medial to both of them. 



The branches given off by the tibial nerve in the popliteal space are articular, 

 cutaneous, and muscular. 



The articular branches are usually three in number, a superior and an inferior 

 internal articular and an azygos articular. They accompany the corresponding 

 arteries, and, after piercing the ligaments, are distributed in the interior of the 

 joint. The superior branch is often wanting. 



The cutaneous branch, the medial sural cutaneous (tibial communicating) 

 nerve, descends between the heads of the gastrocnemius, beneath the deep 

 fascia, to the middle of the calf, where it pierces the fascia and unites with the 

 peroneal anastomotic branch of the lateral sural cutaneous to form the sural 

 (external saphenous) nerve, through which its fibres are distributed to the skin 

 of the lower and dorsal part of the leg and the lateral side of the foot. 



The muscular branches are distributed to both heads of the gastrocnemius, to 

 the plantaris, soleus, and popliteus. 



The nerve to the soleus is relatively large, and passes between the lateral head of the gastroc- 

 nemius and the plantaris before it reaches its termination (fig. 771). The nerve to the 'popliteus 

 descends on the posterior surface of the muscle, turns around its lower border, and is distributed 

 on its anterior aspect. In addition to supplying the popliteus, it gives articular branches to 

 the knee and superior tibio-fibular joints, a branch to the tibia which accompanies the medullary 

 artery, and a long, slender twig which gives filaments to the anterior and posterior tibial arteries, 

 and it descends as the interosseous crural nerve on the interosseous membrane to the inferior 

 tibio-fibular joint. It also gives branches to the interosseous membrane and to the periosteum 

 of the lower part of the tibia. 



Relations. — In the upper part of the leg the tibial nerve is placed deeply, under the gas- 

 trocnemius and soleus, but in the lower half it is merely covered by the deep fascia, which is 

 thickened between the medial maleolus and the calcaneus to form the lacinate (internal annular) 

 ligament, and the termination of the nerve lies either under cover of this ligament, or under the 

 attachment of the abductor hallucis. The anterior relations of the nerve are, from above down- 

 ward, the tibialis posterior, the flexor digitorum longus, the lower part of the tibia, and the pos- 

 terior ligament of the ankle-joint. For a short distance after its commencement the nerve lies 

 to the medial side of the posterior tibial artery; then it crosses behind the artery and runs down- 

 ward along its lateral aspect. 



The branches of the lower part of the tibial nerve (below the popliteal space) 

 are likewise muscular, cutaneous, and articular. They are supplied to the deep 

 muscles of the dorsum of the leg, to the fibula, to the skin of the heel and foot, and 

 to the ankle-joint. Several of the terminal branches are important enough to 

 receive special names and special treatment. 



The muscular branches pass from the upper part of the nerve to the tibialis posterior, 

 flexor digitorum longus, soleus, and flexor hallucis longus. The fibular branch arises with the 

 nerve to the flexor hallucis longus, and accompanies the peroneal artery. It supphes the peri- 

 osteum and gives filaments which accompany the medullary artery. 



The articular branches arise from the lower part of the nerve, immediately above its 

 terminal branches, and they pass into the ankle-joint through the deltoid ligament. 



The medial calcaneal (calcaneo-plantar cutaneous) nerves arise from the trunk of the 

 tibial nerve in the lower part of the leg. They pierce the laciniate (internal annular) ligament, 

 and are distributed to the integument of the medial side and plantar surface of the heel and 

 the adjoining part of the sole of the foot (fig. 771), 



Terminal branches of tibial nerve.— The medial plantar nerve is the larger of 

 the two terminal branches of the tibial nerve. It commences under cover of the 

 lower border of the laciniate (internal annular) ligament, or under the posterior 

 border of the abductor hallucis, and passes forward, accompanied by the small 

 internal plantar artery, in the inter-muscular septum between the abductor 

 hallucis and the flexor digitorum brevis. At the middle of the length of the foot 

 it becomes superficial, in the interval between the two muscles, and divides into 

 four sets of Icrinirial braiichos (fig. 772): — 



(a) Muscular branches ])ass from the trunk of the nerve to the abductor 

 halhicis and t\\o. flcxcjr digitorum l)revis. 



(h) Articular branches are distributed to the talo-navicular (astragalo- 

 sacphoid) and tli(^ naviculari-(;un(;iform'joint. 



(c) Plantar cutaneous branches are supplied to the skin of the medial part of 

 the solo. 



