732 THE NEEVOUS SYSTEM. 



The arrangement of the cutaneous branches of the superficial peroneal nerve is liable 

 to considerable variation. The lateral division of the nerve may be increased in size, and 

 may supply the nerve to the adjacent sides of the second and third toes ; or it may be reduced 

 in size, in which case the nervus suralis takes its place on the dorsum of the foot, often supply- 

 ing as many as two and a half toes on the lateral side. 



The cutaneous nerves on the dorsum of the toes are much smaller than the corresponding 

 plantar digital nerves. They are reinforced on the dorsum of the terminal phalanges by twigs 

 from the plantar nerves, which supply the tips of the toes and the nails. 



NERVUS TIBIALIS. 



The tibial nerve (O.T. internal popliteal) arises from the anterior surface 

 of the sacral plexus, usually from the fourth and fifth lumbar and first three 

 sacral nerves (Fig. 631, p. 736). It is incorporated in the sciatic trunk in the 

 buttock and proximal part of the thigh. At the bifurcation of the sciatic nerve 

 it passes onwards through the popliteal fossa and the back of the leg. The part 

 of the nerve from its origin from the plexus or the bifurcation of the sciatic nerve 

 to the distal border of the popliteus muscle, was formerly called internal popliteal; 

 the part of the nerve in the back of the leg being then designated posterior tibial. 

 The course of the nerve through the buttock and thigh has already been described 

 (p. 728). In the popliteal fossa it is concealed at first by the semimembranosus and 

 the other hamstring muscles. It passes to the medial side of the popliteal vessels, 

 and is thereafter found upon the popliteus muscle, under cover of the gastrocnemius 

 and plantaris. In the back of the leg, from the distal border of the popliteus 

 muscle to the ankle, the tibial (O.T. posterior tibial) nerve lies on the tibialis 

 posterior muscle and the tibia, and, along with the posterior tibial vessels, occupies 

 a sheath in the intermuscular septum separating the superficial and deep muscles 

 of the back of the leg. In the proximal part of the leg the nerve is medial to 

 the vessels, but, crossing behind them, it lies on their lateral side in the distal 

 portion of its course. It terminates under cover of the ligamentum laciniatum by 

 dividing into the lateral and medial plantar nerves. 



The collateral branches may be divided into three series, arising respectively 

 in the region of the thigh, the popliteal fossa, and -the back of the leg : 



(a) Branches arising from the Hoots or Trunk of the Nerve while it is incor- 

 porated with the Sciatic Nerve. These have been already described as muscular 

 branches to the quadra tus femoris, gemelli, obturator internus, and the hamstring 

 muscles, and an articular branch to the hip-joint (Fig. 631, p. 736). 



(b) Branches arising in the Popliteal Fossa proximal to the Knee-Joint, 

 These are in three sets articular, muscular, cutaneous. 



1. The articular branches are slender nerves, variable in number. There are 

 usually two, an azygos branch which pierces the oblique ligament of the knee- 

 joint, and a medial branch, a long fine nerve which, crossing the popliteal vessels, 

 runs distally on the medial side of the fossa to accompany the distal medial articular 

 artery to the knee-joint. In its course it gives off a branch, often absent, which 

 accompanies the proximal medial articular artery. 



2. The muscular branches are five in number. Nerves for the two heads of 

 the gastrocnemius, and for the plantaris enter those muscles at the borders of the 

 popliteal fossa. A nerve for the soleus enters the superficial surface of the 

 muscle. A nerve for the popliteus muscle passes over the surface of that muscle, 

 and after winding round its distal border, supplies it on its deep (anterior) surface. 

 As this nerve passes below the popliteus it supplies branches to the tibialis 

 posterior, an inter osseous branch for the interosseous membrane, which can be 

 traced as far as the tibio-fibular syndesmosis, an articular branch for the 

 proximal tibio-fibular joint, and a medullary branch for the shaft of the tibia. 



3. N. Cutaneous Surae Medialis (O.T. N. Communicans Tibialis). The 

 cutaneous branch is the medial cutaneous nerve of the leg. This nerve passes 

 from the popliteal fossa in the groove between the two heads of the gastrocnemius 

 muscle, and afterwards lies upon the tendo calcaneus. It pierces the deep fascia 

 in the middle third of the back of the leg, and is joined immediately afterwards 

 by the peroneal anastomotic ramus from the common peroneal nerve. From 



