608 THE NERVOUS SYSTEM. 7 
short head of the biceps. When traced up to the plexus, it is associated with the 
back of the fourth and fifth lumbar and first sacral nerves. It passes through the 
upper part of the popliteal space concealed by the biceps muscle, and separates 
into upper and lower branches, which accompany the upper and lower external 
articular arteries to the outer side of the knee-joint. 
Terminal Branches — The peroneal (external popliteal) and tibial (internal 
popliteal) nerves are the two main trunks resulting from the combination of the 
posterior and anterior cords respectively of the sacral plexus. The peroneal nerve 
is homologous with the musculo-spiral nerve in the upper limb; the tibial nerve 
represents a medio-ulnar trunk ; and as already stated, the two nerves, constituting 
the great sciatic nerve, are enveloped in a common sheath for a variable distance 
before beginning an independent course. 
PERONEAL NERVE. 
The peroneal or external popliteal nerve (n. peronzeus communis) arises from 
the back of the sacral plexus from the fourth and fifth lumbar and first two 
sacral nerves. Incorporated with the great sciatic nerve in the buttock and 
upper half of the thigh, it passes downwards from the bifurcation of that nerve 
through the popliteal space to its termination at a point about an inch below the 
head of the fibula. It is concealed at first by the biceps muscle. Following 
the tendon of that muscle, it passes obliquely through the upper and outer part 
of the popliteal space and over the outer head of the gastrocnemius muscle to the 
back of the head of the fibula. In the lower part of its course it is directly beneath 
the deep fascia. 
Collateral Branches.—These are divided into two sets: (a) Those arising from 
the roots or trunk of the nerve while it is in combination with the tibial nerve in 
the great sciatic trunk. These have been already described, viz. a muscular branch 
to the short head of the biceps, and an articular branch to the knee-joint. (b) Those 
arising in the popliteal space. These are cutaneous branches, viz. a sural branch and 
the peroneal communicating. 
The sural branch (n. cutaneus sure lateralis) is irregular in size and distribution, 
and may be represented by two or more branches (Fig. 442, p. 603). Arising from 
the peroneal nerve in the popliteal space, often in common with the succeeding 
nerve, it pierces the deep fascia over the outer head of the gastrocnemius, and 1s 
distributed to the skin on the outer aspect of the back of the leg in the upper 
two-thirds. The extent of its distribution varies with that of the small sciatic 
and external saphenous nerves. 
The peroneal communicating nerve (r. anastomoticus peroneus, r. communicans 
fibularis), arising in the popliteal space, passes over the outer head of the gastro- 
cnemius beneath the deep fascia to the middle third of the leg, where it assists in 
forming the external saphenous nerve by its union with the tibial communicating 
branch of the tibial nerve. In many cases the two branches do not unite. In such 
cases the peroneal communicating nerve may be limited in its distribution to the 
skin of the outer side of the leg, heel, and ankle, or it may be distributed to the 
area usually supplied by the external saphenous nerve. 
Terminal Branches.—The terminal branches of the peroneal nerve are 
recurrent tibial, anterior tibial, and musculo-cutaneous. They arise just below 
the head of the fibula, and are directed forwards, diverging in their course, 
beneath the peroneus longus muscle. 
The recurrent tibial nerve is the smallest branch. Passing forwards beneath 
the origin of the peroneus longus and the extensor longus digitorum muscles, it 
divides below the outer tuberosity of the tibia into branches which supply the 
upper fibres of the tibialis anticus muscle, the tibio-fibular articulations, and the 
knee-joint. 
ANTERIOR TIBIAL. NERVE. 
The anterior tibial nerve (n. peronweus profundus) passes downwards and in- 
wards, beneath the peroneus longus, extensor longus digitorum, and extensor proprius- 
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