THE FIBULA. 231 
of the aponeurosis which covers the tibialis posticus muscle. The remainder of 
the extensor aspect of the shaft, which, above, is directed backwards, is so twisted 
that inferiorly it is directed inwards. From this, in its upper part, the soleus 
muscle arises; whilst lower down, the flexor longus hallucis muscle derives an 
extensive origin. Both of these muscles, together with the tibialis posticus, act as 
extensors of the ankle. On this aspect of the bone, at or near the middle of the 
shaft, and just behind the prominent tibial ridge, is the opening of the nutrient 
canal, which has a downward direction. 
The inferior extremity of the fibula, or external malleolus (malleolus lateralis), 
is of pyramidal form. Its inner surface is furnished with a triangular articular 
area (facies articularis melleoli), plane from before backwards, and slightly convex 
from above downwards, which articulates with a corresponding surface on the outer 
side of the body of the astragalus. Behind this there is a deep pit, to which the 
posterior fasciculus of the external lateral ligament is attached. Above the 
articular facet there is a rough triangular area on the extensor surface of the shaft, 
from the summit of which the interosseous ridge arises; hereto are attached the 
strong fibres of the inferior interosseous ligament which bind together the opposed 
surfaces of the tibia and fibula. The external surface of the inferior extremity 
forms the elevation of the external malleolus which determines the shape of the 
projection of the outer ankle. Rounded from side to side and from above down- 
wards, it terminates below in a pointed process, which reaches a lower level than 
the corresponding process of the tibia, from which it also differs in being narrower 
and more pointed and being placed in a plane nearer the heel. Superiorly, this 
surface, which is subcutaneous, is continuous with the triangular subcutaneous area 
so clearly defined by the convergence above of the lines which unite to form the 
anterior border. The anterior border and tip of the external malleolus furnish 
attachments to the anterior and middle bands of the external lateral ligament of 
the ankle. The posterior surface of the external malleolus, broad above, where 
it is confluent with the peroneal or external surface, is reduced in width below 
by the presence of the pit which hes to its inner side. This aspect of the 
bone is grooved (sulcus malleolaris) by the tendons of the peroneus longus and 
brevis muscles, which curve round the posterior and lower-pointed aspect of the 
malleolus. ‘The proportionate length of the fibula to the body height is as 1 is to 
4:37-4:82. 
Arterial Foramina.—Numerous minute vascular canals are seen piercing the outer surface of 
the head, and one or two of larger size are seen on the inner surface immediately in front of the 
superior articular facet. The canal for the nutrient artery of the shaft, which has a downward 
direction, is situated on the back of the bone about its middle. The outer surface of the external 
malleolus displays the openings of many small canals, and one or two larger openings are to be 
noted at the bottom of the pit behind the inferior articular surface. 
Connexions.—The head and external malleolus, and part of the shaft immediately above the 
latter, are subcutaneous. The remainder of the shaft is covered on all sides by the muscles which 
surround it. Superiorly the bone plays no part in the formation of the knee-joint, but inferiorly 
assists materially in strengthening the ankle-joint by its union with the tibia and its articulation 
with the astragalus. In position the bone is not parallel to the axis of the tibia, but oblique to 
it, its upper extremity lying posterior and external to a vertical line passing through the external 
malleolus. 
Architecture.—A medullary canal runs throughout the length of the shaft, reaching as high 
as the neck above, and extending as low as a point about 24 inches above the inferior extremity 
of the external malleolus. The outer wall of the shaft is usually considerably thicker than the 
inner. The head is formed of loose cellular bone, enclosed within a very thin dense envelope. 'The 
spongy tissue of the lower extremity is more compact, and acquires considerable density on the 
surfaces underlying the articular area and the pit behind it. The canal for the nutrient artery of 
the shaft opens into the medullary cavity about an inch below its external aperture. 
Ossification.—The shaft begins to ossify about the middle of the second month of 
foetal life. At the end of the third month there is but little difference in size between it 
and the tibia, and at birth the fibula is much larger in proportion to the size of the tibia 
than in the adult. Its extremities are cartilaginous, the lower extremity not being as long 
as the internal malleolar cartilage of the tibia. It is in this, however, that an ossific centre 
first appears about the end of the second year, which increases rapidly in size, and unites 
with the shaft about nineteen years. The centre for the superior epiphysis begins to 
