BONES OF THE FOOT. 



339 



required for prehension have the inferior seg- 

 ments of all four extremities organised as hands, 

 and are thence denominated Quadrumanous. 



The most elevated part of the foot is at its 

 posterior part, where it contributes to form the 

 ankle-joint; thence it inclines forwards, gra- 

 dually expanding transversely, and presenting 

 a more or less convex surface from behind 

 forwards. This is the dorsum pedis, the instep. 

 The inferior surface likewise expands as it 

 proceeds forwards. It is slightly concave in 

 the transverse direction, and more manifestly 

 so in the antero-posterior one ; this latter, how- 

 ever, varies in a degree proportionate to the 

 convexity of the dorsum. This is the plant a 

 pedis, the sole. 



The internal edge of the foot corresponds to 

 the great toe, the external edge to the little toe, 

 the anterior to the ends of the toes, and the 

 posterior extremity of the foot is formed by the 

 os calcis. 



I. Tarsus (Germ, die Fusswurzel). Nearly 

 the posterior half of the foot is occupied by 

 the tarsus, which is arranged in the form of an 

 arch, convex superiorly, on the highest point 

 of which rests the weight of the leg. Seven 

 bones enter into the formation of the tarsus ; 

 they are arranged in two sets or rows. The 

 posterior row is formed by the astragalus and 

 os calcis, the anterior row by the os naviculare, 

 the os cuboidcutjii and the three cuneiform 

 bones. Through the medium of the first two 

 bones of the anterior row that row is articulated 

 with the posterior. 



1 . Astragalus (ac-r^aya^o?, Ttrgugoq, os ba- 

 listfE, talus; Fr. Vastragale ; Germ, das Knoc/i- 

 elbe.in oder Sprungbein). 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, 

 planiform, 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 

 fur the lodgement of a strong ligament which 

 passes between the astragalus and os calcis. 



All that portion which is behind the neck 

 constitutes what is called the body, on which 

 we notice five 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 in wards, and is destined to lodge 

 the tendon of the flexor pollicis longus. 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 th os calcis. Immediately 

 in front of it there is a deep and narrow depres- 

 sion which separates it from an oval planiform 

 facet for articulation with the sustentaculum of 

 the os calcis. 



2. Os calcis (wrtqva, o-xeXi? ; Fr. le calca- 

 neum, os du talon; Germ, das Fersenbein ; the 

 heel-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 astralagus 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 processus interims, or sus- 

 tentaculum cervicis tali of Albinus ; it supports 

 and is articulated with a corresponding facet 

 on the under surface of the neck of the astra- 

 galus. A narrow groove on the outside of the 



* See further description in the article ANKLK- 

 JOIKT. 



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