2:96 . THE ARTICULATIONS OR JOINTS. 
A synovial membrane is found lining the small joint-cavity, but it is always a 
direct prolongation from that which lines the ankle-joint. 
JOENTS OF THE HOOT: 
THE ANKLE-JOINT. 
The ankle-joint (articulatio talo-cruralis) is a ginglymus variety of a diarthrosis. 
The bones which enter into its formation are the lower ends of the tibia and fibula, 
with the articular areas on the upper, lateral, and inner surfaces of the astragalus. 
The tibia and fibula, aided by the transverse inferior tibio-fibular Ligament, form a 
three-sided socket within which the astragalus is accommodated. The roof or 
highest part of the socket, which is wider in front than behind, is formed chiefly by 
the quadrilateral articular surface which characterises the lower end of the tibia, 
but towards its postero-external margin the transverse inferior tibio-fibular liga- 
ment assists in its formation. Here also the tibial articular surface is continuous 
with the narrow articular facet already described as forming part of the inferior 
tibio-fibular joint. The inner wall of the socket is formed by the articular facet on 
the outer side of the internal malleolus, and there is no interruption of the articular 
cartilage between the roof and inner wall. The outer wall of the socket is quite 
separate from the foregoing parts, and consists of a large triangular facet upon the 
inner side of the external malleolus. This facet is situated immediately in front of 
the deep pit which characterises the posterior part of this surface of the fibula. 
A small lunated facet is frequently found upon the anterior surface of the lower end of the 
tibia, particularly among those races characterised by the adoption of the “squatting” posture. 
When this facet. exists it is continuous with the anterior margin of the roof of the socket, and it 
articulates with a similar facet upon the upper surface of the neck of the astragalus in the 
extreme flexion of the ankle-joint which “squatting” entails. 
The articular surface upon the body of the astragalus adapts itself to the tibio- 
fibular socket, and presents articular facets corresponding to the roof and sides of 
the socket. Thus the superior surface of the astragalus possesses a quadrilateral 
articular area, wider in front than behind, distinctly convex in the antero-posterior 
direction, and slightly concave transversely. In addition, towards its postero- 
external margin, there is also a narrow antero-posterior facet corresponding to the 
transverse inferior tibio-fibular ligament. The articular cartilage of this upper 
surface is continued without interruption to the tibial and fibular sides of the bone, 
although the margins of the superior area are sharply defined from the lateral 
facets, the outer of which is triangular in outline, while the inner is pyriform, but 
in each case the surface 1s vertical. 
Ligaments.—The ligaments form a complete investment for the joint, @.e. a 
capsule in which the individual parts vary considerably in strength, and are 
described under separate names. . 
The anterior ligament (Fig. 229) is an extremely thin membrane, containing 
very few longitudinal fibres. It extends from the lower border of the tibia to the 
upper border of the head of 
the astragalus, passing in 
_ fo} “ . 
Internal. front of a pad of fat which 
lateral or 
deltoid fills up the hollow above the 
\ ankle-joint neck of that bone. 
The posterior ligament (Fig. 
226) is attached to -con- 
tiguous non-articular borders 
of the tibia and astragalus. 
~ Internal 3 Ose 
>t - malleolus Many of its fibres radiate 
Posterior talo-fibular Transverse inferior “Synovial pad of fat inwards from the external 
iva ibio-fibular ligament 5 
ligament tibio-fibular ligamen malleolus. This aspect of the 
Fia. 227.—ARTICULAR SURFACES OF rs AND FIBULA WHICH" joint is strengthened by the 
ARE OPPOSED TO THE ASTRAGALUS. 
strong, well-defined, trans- 
verse ligament already described in connection with the inferior tibio-fibular joint. 
Anterior talo- Anterior inferior tibio-fibular ligament 
fibular ligament 
Articular facet on 
external malleolus 
A 
External ys 
malleolus ‘ 
Caleaneo-fibular 
ligament 
Posterior inferior 
tibio-fibular 
ligament 
The external lateral ligament (Figs. 227 and 229) is very powerful, and is divisible 
