610 THE NERVOUS SYSTEM. 4 
The external terminal branch of the musculo-cutaneous nerve descends over the 
anterior annular ligament, and after supplying branches to the lower part of the 
lee and the dorsum of the foot, divides into two parts, internal and external, which, 
passing to the intervals between the third and fourth, and fourth and fifth toes 
respectively, divide into dorsal digital branches for the adjacent sides of these toes. 
These branches communicate with offsets of the external saphenous nerve. 
The arrangement of the cutaneous branches of the musculo-cutaneous is Hable to considerable 
variation. The nerve to the adjacent sides of the second and third toes may come from the 
outer division of the nerve, which, again, is not unfrequently much reduced in size, in which 
case the external saphenous nerve takes its place on the dorsum of the foot, often supplying as 
many as two and a half toes on the outer side. 
The cutaneous nerves on the dorsum of the toes from the anterior tibial and musculo-cutaneous 
nerves 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 nails. 
TIBIAL NERVE. 
The tibial or internal popliteal nerve (n. tibialis) arises from the front of 
the sacral plexus, usually from the fourth and fifth lumbar and first three sacral 
nerves (Fig. 446, p. 614). It is incorporated in the great sciatic trunk in the 
buttock and upper part of the thigh. At the bifurcation of this nerve it passes 
directly downwards through the popliteal space, and enters the back of the leg at 
the lower border of the popliteus muscle. The part of the nerve from its origin 
from the plexus or the bifurcation of the great sciatic nerve to the lower border of 
the popliteus muscle is sometimes called internal popliteal; the part of the nerve in 
the back of the leg being then designated posterior tibial. In the popliteal space 
it is concealed at first by the semimembranosus and other hamstring muscles. It 
afterwards lies beneath the gastrocnemius and plantaris muscles, and in its course 
it crosses the popliteal vessels from without inwards, so as to lie below upon the 
popliteus muscle. In the back of the leg, from the lower border of the popliteus 
muscle to the ankle, the tibial (or posterior tibial) nerve les on the tibialis posticus 
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 upper part of the leg it is internal to the vessels, but, crossing 
over them, it hes on their outer side in the lower portion of its course. The nerve 
terminates beneath the internal annular ligament by dividing into the external 
and internal plantar nerves. 
The collateral branches may be divided into three sets :— 
(a) Branches arising from the Roots or Trunk of the Nerve while vt is incor- 
porated with the Great Sciatic Nerve-—These have been already described, viz. 
muscular branches to the quadratus femoris, gemelli, obturator internus, and the 
hamstring muscles, and an articular branch to the hip-joint (Fig. 446, p. 614). 
(b) Branches arising in the Popliteal Space above 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 posterior ligament of the knee 
joint, and an internal branch, a long fine nerve which, crossing the popliteal vessels, 
descends on the inner side of the space to accompany the lower internal articular 
artery to the knee-joint. In its course it gives off a branch which accompanies the 
upper internal articular artery. It is often absent. 
The muscular branches are five in number. Nerves for the two heads of the 
gastrocnemius and plantaris enter the muscles at the borders of the popliteal space. 
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 
lower border, supplies it on its deep (anterior) surface. As this nerve passes beneath 
the poplteus it supplies branches to the tibialis posticus muscle, an interosseous 
branch for the interosseous membrane, which ean be traced as far as the lower 
tibio-fibular articulation, an articular branch for the upper tibio-fibular joint, and 
a medullary branch for the shaft of the tibia. 
3. The cutaneous branch is the tibial communicating nerve (n. communicans 
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