a 
: INTERTARSAL JOINTS, 299 
the external tubercle of the astragalus to the upper surface of the os calcis, 
immediately behind the articular facet. 
The internal talo-caleaneal ligament (Fig. 228) lies obliquely on the inner side 
of the joint, and consists of fibres which extend from the inner posterior tubercle 
of the astragalus to the hinder roughened border of the sustentaculum tali. Some 
of its fibres become continuous with the internal calcaneo-scaphoid ligament. 
The interosseous talo-calcaneal ligament (Fig. 229) closes the antero-internal 
aspect of the joint. It is the strongest of the series of ligaments entering into the 
capsule. Compared with it the other bands are, comparatively speaking, insigni- 
ficant. Its attachments are to the bottom of each groove, so that it occupies the 
tarsal canal formed by these opposing grooves. 
A synovial membrane lines the capsule, and it is distinct from other tarsal 
synovial membranes, 
Articulatio Talo-calcaneo-navicularis.—This is one of the most important of 
the joints of the foot, not only because the astragalus is here situated in relation 
to the summit of the antero-posterior arch of the foot, but because the head of 
the astragalus is received into a composite socket made up of sustentaculum tali, 
scaphoid, and the inferior or internal calcaneo-scaphoid ligament. 
The articular surface on the head of the astragalus presents anteriorly a 
convex rounded facet for articulation with the scaphoid, inferiorly a convex facet 
which rests upon the sustentaculum tali, and intermediate between these two there 
is a triangular facet which articulates with the inferior calcaneo-scaphoid ligament. 
All these facets are in continuity with each other, and are in front of the tarsal 
groove on the under surface of the astragalus. Occasionally a fourth narrow facet is 
found along the outer and hinder part ‘of the articular surface of the head of the 
astragalus, \ whereby it articulates with superior or external calcaneo-scaphoid ligament. 
The scaphoid or navicular bone presents a shallow, cup-shaped, articular cavity 
towards the head of the astragalus. 
The articular surface of the sustentaculum tali is concave, and is usually marked 
off into two facets. 
Two ligaments play an important part in binding together the os calcis and 
the scaphoid, although these bones do not directly articulate; and further, these 
ligaments provide additional articular surfaces for the head of the astragalus. 
These are the two following :— 
(a) The inferior or internal caleaneo- scaphoid ligament (Figs. 228 and 230) is an 
extremely powerful fibro-cartilaginous tie-band. It extends between the anterior 
margin of the sustentaculum tali and the inferior surface of the scaphoid bone. Certain 
of its upper fibres radiate upwards on the inner surface of the scaphoid, and become 
continuous with the tibio-navicular portion of the deltoid ligament of the ankle- 
jomt. The plantar aspect of this hgament is in contact with the tendon of the 
tibialis posticus muscle, through which the head of the talus receives great support. 
Superiorly it contributes an articular surface which forms a triangular portion of 
the floor of the composite socket in which the head of the talus is received. 
(6) The superior or external calcaneo-scaphoid ligament (Fig. 229) lies deeply 
in the front part of the sinus tarsi, 7.e. the interval between the astragalus and os 
calcis. Its fibres are short, and extend from the dorsal surface of the front part of 
the os calcis, immediately to the outer side of the sustentacular facet, forwards to 
the outer side of the scaphoid bone. Frequently the ligament presents a surface 
which articulates with the head of the astragalus, and in these cases it forms a part 
of the composite socket. 
The cavity of the talo-calcaneo-navicular joint is closed posteriorly by the 
interosseous talo-calcaneal ligament already described. On its inner and outer 
inferior aspects it is closed by the calcaneo-scaphoid ligaments. 
The superior and lateral aspects are covered by an astragalo-scaphoid membrane 
or ligament. This hgament is thin, and extends from the upper non-articular area 
on the head of the astragalus to the dorsal surface of the scaphoid bone. It may 
be subdivided into dorsal (superior), /ateral (external), and medial (internal). 
astragalo-scaphoid ligaments (Fig. 228), which, with the calcaneo-scaphoid and 
interosseous talo-caleaneal ligaments, complete the capsular investment of the joint. 
