1. "him:galas (currectycaoc, Tergeogoc, as ba lista-, talus ; l'astrag,ak ; Germ. das Knoch akin oder Sprun,gbein).—This bone is situated between the tibia and the os calcis, and has the navicular bone in front of it. In point of size it ranks second among the tarsal bones, the os calcis being first.
The astragalus is commonly divided into three parts for the purposes of description, viz. the head, neck, and body. The head is that convex portion which forms the anterior part of' the bone, and which is entirely articular. This smooth, oval, articular head is adapted to the posterior concavity of the navicular bone. The aspect of this surface is forwards, inwards, and slightly downwards. On the inferior part of the head we notice another articular facet, planiforin, situated internally, and generally con tinuous with the articular surface last described. By means of this facet the astragalus moves on a corresponding surface on the upper and anterior part of the os calcis.
The head of the astragalus is connected to the body by a narrow contracted portion called the neck, which is rough on all its surfaces, giving insertion to ligaments and perforated by numerous foramina for the transmission of vessels. The external side of the neck pre sents a remarkable excavation, which affords insertion to and contributes to bound a space for the lodgement of a strong ligament which passes between the astragalus and os All that portion which is behind the neck constitutes what is called the body, on which we notice live surfaces. a. The superior sur face forms an articular trochlea, convex from before backwards, and slightly concave trans versely; it articulates with the inferior extremity of the tibia :• immediately in front of it there is a roughness of very limited extent, which affords insertion to ligamentous fibres. b. The posterior surface is almost wholly occupied by a well-marked groove, which passes obliquely downwards and inwards,and is destined to lodge the tendon of the flexor pollicis longs. c. The external surface is occupied by a triangular facet, whose base is direct upwards and is con tinuous with the articular part of the superior surface of the body ; this facet articulates with the fibula. It is bounded below and behind by a rough portion for ligamentous insertion. d. The internal surface is also articular in its upper half for the adaptation of the inner malleolus : it, too, is triangular, and by its base is continuous with the superior surface. Below this internal malleolar facet the bone is rough and irregular, and here the internal lateral ligament of the ankle-joint is inserted. Lastly, the inferior surface is occupied almost entirely by a concave articular facet, oval, with its long axis directed from within outwards and forwards; this facet is articulated with a corre sponding one upon the os colds. Immediately in front of it there is a deep and narrow depression which separates it from an oval planiform facet for articulation with the sustentaculum of the os calcis.
2. Os calcis (wrsect, asesic; Fr. le eaten scum, os du talon ; Germ. dos Fersenbein ; the hcel-bone).—This is the largest bone of the tarsus; it occupies the most posterior part of the foot, and is situated immediately under neath the astragalus, of which it constitutes the principal support. Its greatest extent is from before backwards. It is somewhat flattened on the sides: its direction is horizontal, the foot in standing resting upon the most posterior part of its inferior surface. This horizontal direc tion of the heel-bone is one of the arguments which anatomy affords in support of the asser tion that the erect posture is natural to man.
We notice six surfaces upon this bone.
a. The superior surface, or that upon which the astralagns rests. On it we observe in front three articular facets, separated from each other by distinct intervals: the first or smallest is situated at the anterior edge of the surface and at its internal angle, and is articulated with the facet on the inferior part of the head of the astragalus ; it is not constant. The second is posterior and internal to the last, separated from it by a rough depression about a quarter of an inch in extent. This is oval, slightly con cave, and is marked upon a projecting portion of the bone which overhangs the anterior part of the internal surface, and which is known under the name of processes internus, or sus tcntaealam cervicis tali of Albinus; it supports and is articulated with a corresponding Lett on the under surface of the neck of the astra gains. A narrow groove on the outside of the facet last named separates it from the third and largest one; this is articulated with the facet which is on the inferior surface of the body of the astragalus; it is oval, convex, and its long axis directed forwards and outwards. Imme diately in front of this articular facet there is a hollow, rough, non-articular surface for the insertion of the ligament which connects the astragalus to the os calcis, and behind the facet the remaining portion of the superior surface of the bone is also non-articular, slightly exca vated from before backwards, varying in length in different subjects, and on this variety de pends the diversity in the length of the heel. b. The posterior surface, oval in its outline, rough and fibrous in its inferior half, where the tendo Achillis is inserted, smooth in its supe rior half where a bursa is placed, over which the tendon glides. c. The iqferior or plantar surface, nearly equal in extent to the superior, and in the natural position directed obliquely upward and forwards. Here we find, in ex amining the parts from behind forwards, first, two tubercles, upon which the heel rests in standing, and which seem peculiarly to cha racterize the human heel-bone. These tuber cles are separated from each other by a depres sion; the internal one is greatly the larger—it affords attachment to the short flexor of the toes ; the external one is small and pointed, and to it are attached the abductor minimi digiti muscle and the plantar fascia. Secondly, in front of these tubercles the bone is very rough and flat to within half an inch of its anterior margin, where it is slightly grooved transversely. The whole of this portion gives insertion to the strong calcaneo-cuboid ligament. d. The ante rior or euboid surface, which is entirely articu lar, triangular, with its base upwards, slightly concave, and articulated with the cuboid bone. e. The external surface, quite subcutaneous, so that here the bone is greatly exposed to injury, and may be easily got at for surgical operation. It is slightly convex, its posterior half being double the size of the anterior in ver tical measurement; at the anterior part of the former there are two superficial grooves directed obliquely forwards and downwards, separated by a slightly prominent tubercle. The anterior of these grooves gives passage to the tendon of the peroneus brevis, the posterior to that of the peroneus longus. f. The internal surface, excavated in its whole extent, lodges the ten dons and nerves which are passing from the back of the leg to the sole of the foot; at the junction of its anterior and posterior halves it is overlapped by the sustentaculuin, the inferior surface of which is grooved by the tendon of the long flexor of the great toe.