1 66 Anatomy of Skeleton 



this corresponds with the differences in the appearances of the lines of attachment of 

 the aponeurosis to the bones : the outer line forms the prominent postero-internal 

 border of the fibula, whereas the inner attachment makes the badly-marked vertical 

 line on the tibia. These two lines separate the area of Tibialis posticus on each bone 

 from the area of the corresponding long flexor, so the area for Flexor longus hallucis on 

 the fibula is immediately behind the postero-internal border, on the posterior surface 

 of the bone. 



Tibialis posticus passes downwards and inwards from its origin (Fig. 134) across 

 the tibia, and thus leaves the region of the interosseous membrane. In this way the 

 aponeurosis that covers it comes into relation with the membrane as the muscle inclines 

 away from it, and so we get the explanation of the postero-internal ridge running below 

 into the interosseous border : it marks the lower end of origin of Tibialis posticus, and 

 comparison of a few bones will make it evident that the extent of this origin and the 

 consequent position of the junction of the two ridges is very variable. Moreover, the 

 size of the muscle does not vary with the general muscularity of the individual. 



Now examine the flexor surfaces of the fibula, and it is clearly apparent that the 

 Tibialis posticus takes the whole of the inner aspect of the surface in the middle of the 

 shaft and Flexor longus hallucis is altogether on the posterior aspect. But as we follow 

 the surfaces down we find the Tibialis area narrowing until it disappears, and, as it 

 narrows, the area for the long flexor, keeping close to it, gets more and more on the 

 inner side, so that in the lower part it is altogether on the inner aspect -in other words, 

 the long flexor, arising partly from the aponeurotic covering of the Tibialis posticus, 

 is carried by this to the interosseous membrane when the latter muscle leaves it 

 uncovered, and is in this way brought to the inner side of the shaft. If the 

 Tibialis is small the long flexor comes to the inner side comparatively high up and 

 acquires an origin from the membrane, but under opposite circumstances it may not 

 do so. 



Here we have the explanation of the twisted appearance of the bone, and at the 

 same time we can see the " curve " of the twist, and therefore the appearance of any 

 individual bone, depends really on the size of the Tibialis posticus. As soon as this 

 muscle leaves the fibula the Flexor longus hallucis takes its place on the inner side of 

 the bone, and thus affords opportunity for the peroneal tendons to come on the back 

 of the bone so that they may pass behind the malleolus to obtain their proper action 

 on the foot : because the bone is moulded by the muscles we therefore find that its 

 posterior surface, traced down, becomes internal, and its outer surface can be followed 

 down to the groove behind the malleolus. 



The extensors of the foot arise from the bone immediately internal to the Peronei, 

 that is, from the anterior surface between the anterior edge and the interosseous line. 

 Follow the anterior surface down : when the Peronei move round towards the back 

 of the bone, away from its outer aspect, the extensors do not follow them but remain 

 on its front surface, and thus the lower part of the outer aspect is left uncovered and 

 becomes the triangular " subcutaneous area." 



If the foregoing account of the way in which the surfaces are disposed on the 

 fibula is thoroughly understood there can be no difficulty in following the minor 

 details on the bone. It is better to deal first with the muscles (Fig. 137). 



Tibialis Posticus. The area is well defined by the interosseous line and postero- 

 internal border except at the top, where the two structures which make the lines are 

 continuous as thin fasciae that usually leave no marking on the bone : the area is not 

 limited in any way by the oblique lines, which are included in it, being the attachments 

 of intramuscxilar tendons. 



