176 Anatomy of- Skeleton 



gains (Fig. 145) : a marking on the outer and upper aspect of the neck is for the anterior 

 band, while the posterior fasciculus reaches a groove that lies on the back and outer 

 aspect of the bone, and a small nodule of bone that lies at the end of this and may 

 occasionally exist as a separate ossicle, the os trigonum. This band, as seen in Fig. 136, 

 lies below the level of the transverse inferior tibio-fibular band, which is covered by 

 synovial membrane on its deep aspect, and comes into relation with the astragalus, 

 causing the postero-external part of the trochlear surface to show a slightly bevelled-off 

 facet for it (Fig. 145). 



Now consider the action of the lateral ligaments in movement of the joint. The 

 trochlear surface forms an arc of 120 degrees of a circle with a radius of about four-fifths 

 of an inch. The articular surface is about half as long again as that on the tibia, so 

 that there is not a great amount of movement possible between the bones. The centre 

 of rotation, roughly, passes through the apex of the outer malleolus and below and 

 behind the inner malleolus : the effect of movements on the various bands is considered 

 with Fig. 145. 



FIG. 145. Left astragalus seen from the sides, from above, and from behind. A, on the upper surface of 

 the neck of the bone, is an anterior prolongation of the synovial cavity of the ankle which is well marked 

 in the new-born foot ; in the adult it is as a rule reduced and broken up into loculi by the presence of 

 fibro-fatty septa which project into it. The extension of synovial cavities beyond their articular 

 surfaces is particularly marked in this joint and in the astragalo-scaphoid articulation, and on the inner 

 side of the neck the two synovial membranes approach each other fairly closely, being separated only 

 by thin and indefinite capsular fibres. The articular surface for the tibia is about 4 mm. broader in 

 front than behind ; in dorsi-flexion this broadening causes the astragalus to be locked firmly between, 

 the bones of the leg, but in dorsi-extension the narrower part of the astragalus is engaged and it is 

 supposed that a certain small amount of lateral movement is then possible. The axis of movement 

 of the ankle-joint passes approximately through the end of the fibular malleolus, and below and behind 

 the inner malleolus ; thus the middle fasciculus of the external lateral ligament can be more or less 

 tense throughout the range of movement, which is not large but rather less than 55. But the anterior 

 and posterior fasciculi are attached to the fibula further away from the axis, so they are of more value 

 in limiting movement, and they become tense in dorsi-extension and dorsi-flexion respectively. The 

 fibres of the internal ligament are strongest in the middle and posterior parts, the latter especially 

 having a large and well-marked area of attachment on the astragalus ; these fibres limit dorsi-extension 

 as, in that movement, the malleolus of the tibia is carried forward on the astragalus. The other liga- 

 ments on the bone are attached round the articular surfaces concerned. The line of the centre of gravity 

 of the body falls on or in front of the front part of the ankle-joint in the standing position. 



FIG. 147. Various surfaces of left os calcis. On the outer surface, A is an area covered by the middle 

 fasciculus of the external lateral ligament of the ankle passing down to the tubercle below it ; the pero- 

 neal tendons turn forward over this fasciculus, being held down by a sling-like annular band that is 

 attached to the bone below the ligamentous tubercle. B, surface covered by subcutaneous tissues. 

 C, on the upper aspect, is in relation with the loose fatty tissue lying between the tendo Achillis and the 

 ankle. The upper right figure shows the back surface of the bone, on which are three areas ; upper for 

 the bursa deep to the tendo Achillis, middle for this tendon, and lower roughened by the attachment 

 of the thick fibro-fatty tissue of the heel. 



In addition to the points already seen on the astragalus in connection with the 

 ankle-joint, certain other details are to be noted on the bones. The posterior aspect 

 exhibits two grooves ; one has been already examined, but the other, internal to it, 

 is better marked and broader and is for the tendon of Flexor longus hallucis. When the 

 astragalus and os calcis are articulated it can be seen that this groove leads directly 

 to the groove under the sustentaculum tali. At the top of this groove is the 

 posterior tubercle (or os trigonum) on the outside and a sharp point on the inner side, 

 to which a strong band of the inferior calcaneo-astragaloid capsular ligament is 

 fastened. 



The neck has markings for astragalo-scaphoid dorsal ligaments in addition to 

 lateral fibres. It also affords attachment to interosseous and anterior annular liga- 

 ments (see p. 178). 



