The muscle areas are as shown in Fig. 137 : the upper one reaches the tibio-fibular ligaments above and passes along them to the tibia. There are not usually any indications on the bone of the distinct areas, although there are frequently slight roughnesses for short tendinous fibres in the muscles. Notice that the divisions of the external popliteal nerve lie in the substance of the upper muscle, practically on the bone, and the musculo-cutaneous nerve comes forward between the two muscles, being separated from the bone by Peroneus brevis.
Extensor longus digitorum and Peroneus tcrtius arise from the narrow anterior surface, from the ligamentous markings at the upper end to a point easily seen about an inch above the anterior prominence of the lower end. The two muscles have a continuous origin, the division made between them being purely an artificial one. They occupy the whole of the anterior surface, except in the middle two-fourths, where Extensor longus hallucis arises from the surface between them and the interosseous hne. Above the origin of this muscle the attachment of the long extensor of the digits is interrupted by the passage of the anterior tibial nerve.
The Extensors and Peronei are divisions of one mass of muscle tissue, the dorsal extensors of the foot. At an early stage they are included in the same cellular layer (representing enclosing fascia-), but are separated later by the growth of the outer malleolus. Thus the enclosing layer is stranded on the surface of the subcutaneous area and covers it with a thick, felted fibrous membrane.
The deep transverse fascia of the leg separates the Soleus from the deep flexors and main vessels and nerves. Its attachment has already been followed on the tibia (p. 164) up to the fibrous arch over the vessels : from this it passes to the inner side of the head of the fibula and runs down (Fig. 137), separating the Soleus at first from Tibialis posticus, but then passing obliquely across the back of the bone to cover the Flexor longus hallucis, and thus separating Soleus from this muscle. This oblique line may occasionally be made out on the bone, but it is obscured as a rule by the fusion of the fascia with the aponeurotic fibres of the muscles between which it passes.
At the outer side of the long flexor the fascia blends with the posterior peroneal septum, and the two are attached together to the postero-external edge as far as the lower end, where they separate again to follow their respective muscle groups (Fig. 137).
The peroneal artery comes into relation with the fibula as it runs down on the aponeurotic covering of Tibialis posticus. There is frequently an indication of the course of this artery in the form of a slight groove in the bone (Fig. 137).
The artery follows the aponeurosis, and thus ultimately amis on to the interosseous membrane, where it divides into its anterior and posterior divisions : therefore the height at which the anterior artery appears on the front of the leg depends largely on the size of the Tibiahs posticus. The posterior branch runs down deep to the long flexor to emerge from under its outer border to pass behind the malleolus : it must therefore pierce the attachment of the deep transverse fascia here, and when the line of this fascia is definite there is usually a depression in it that marks the passage of the vessels (Fig. 137, 1).
The lower end has tuberculated prominences in front and behind, at the level of the top border of its articular surface, for the anterior and posterior tibio-fibular ligaments (Fig. 136). Below the anterior mass is another marking, on the malleolus, for the anterior band of the external lateral ligament ; the other bands are attached to the margin of the digital fossa, the concavity of which is filled by a fibro-fatty pad and a synovial prolongation from the joint (see Fig. 137).
The groove on the back of the malleolus is lined by a synovial sheath for the peroneal tendons.
Notice that the articular surface has its long axis almost vertically placed, thus differing from the corresponding surface on the tibial malleolus : it is also longer than the inner malleolus, a development probably associated with the upright position.
The epiphysial line of the lower end lies above the articular surface.
This bone has the " growing end " at its upper extremity, and it is an exception to the rule that the centres for the growing end appear first, for the centre for the lower end comes as a rule some little time before that for the upper end : the latter, however, unites last-otherwise it would not be the " growing end".
Now consider the bone as a whole. The shaft is very exceptionally straight : it is nearly always bowed in a curve with the concavity forwards. It does not carry weight, so does not require the great strength of the tibia. An examination of the bone shows that there is a primary cylindrical bar running through it, covered in and largely hidden by the moulding of the muscles, so that the various surfaces and ridges may be mainly considered as secondary in nature. The primary bar is most evident in the lower half of the peroneal surface, running down to the malleolus : in the upper half of the bone it lies near its back part, and the secondary production of the bone in front of it leads to the concavities found here on the inner and outer surfaces.
The shaft of the fibula, in its middle portion, is just behind the plane-level of the posterior surface of the tibia. The head can be felt distinctly, as can also the prominent lower malleolus, but the upper two-thirds or so of the shaft is only indirectly palpable through the mass of the peroneal muscles. The end of the external popliteal nerve can be felt, in the substance of the Peroneus longus, against the neck of the bone. The line of the tibio-fibular articulation can sometimes be distinctly felt just above the ankle, immediately internal to the lower end of the fibula and external to the common mass of extensor and Peroneus tertius tendons.