172 OSTEOLOGY. 



foot, serving to transmit the weight of the body to the ground, and forming a 

 strong lever for the muscles of the calf. It presents for examination six surfaces ; 

 superior, inferior, external, internal, anterior, and posterior. 



The superior surface is formed behind, of the upper aspect of that part of the 

 os calcis which projects backwards to form the heel. It varies in length in 

 different individuals ; is convex from side to side, concave from before backwards, 

 and corresponds above to a mass of adipose substance placed in front of the tendo 

 Achillis. In the middle of this surface are two, sometimes three, articular 

 facets, separated by a broad shallow groove, directed obliquely forwards and 

 outwards, and rough for the attachment of the interosseous ligament connecting 

 the astragalus and os calcis. Of these two articular surfaces, the external is the 

 larger, and situated on the body of the bone; it is of an oblong form, wider 

 behind than in front, and convex from before backwards. The internal articular 

 surface is supported on a projecting process of bone, called the lesser process of th.e 

 calcaneum (sustentaculum tali); it is of an oblong form, concave longitudinally, 

 and sometimes subdivided into two, which differ in size and shape. More ante- 

 riorly is seen the upper surface of the greater process, marked by a rough 

 depression for the attachment of numerous ligaments, and the Extensor brevis 

 digitorum muscle. 



The inferior surface is narrow, rough, uneven, wider behind than in front, and 

 convex from side to side ; it is bounded posteriorly by two tubercles, separated 

 by a rough depression: the external, small, prominent, and rounded, gives attach- 

 ment to a part of the Abductor minimi digiti ; the internal, broader and larger, 

 for the support of the heel, gives attachment, by its prominent inner margin, to 

 the Abductor pollicis, and in front to the Flexor brevis digitorum muscles; the 

 depression between the tubercles attaches the Abductor minimi digiti, and plantar 

 fascia. The rough surface in front of the tubercles gives attachment to the long 

 plantar ligament ; and to a prominent tubercle nearer the anterior .part of the bone, 

 as well as to .a transverse groove in front of it', is attached the short plantar ligament. 



The external surface is broad, flat, and almost subcutaneous; it presents near 

 its centre a tubercle, for the attachment of the middle fasciculus of the external 

 lateral ligament. Behind the tubercle is a broad smooth surface, giving attach- 

 ment, at its upper and anterior part, to the external astragalo- calcanean ligament ; 

 and in front of the tubercle a narrow surface marked by two oblique grooves, 

 separated by an elevated ridge: the superior groove transmits the tendon of the 

 Peroneus brevis; the inferior, the tendon of the Peroneus longus; the intervening 

 ridge gives attachment to a prolongation from the external annular ligament. 



The internal surface presents a deep concavity, directed obliquely downwards 

 and forwards, for the transmission of the plantar vessels ard nerves and Flexor 

 tendons into the sole of -the foot; it affords attachment to part of the Flexor 

 accessorius muscle. This surface presents an eminence of ! ><me, the lesser process, 

 which projects horizontally inwards from its upper and 1 re part. This process 

 is concave above, and supports the anterior articular surface of the astragalus ; 

 below, it is convex, and grooved for the tendon of the Flexor longus pollicis. 

 Its free margin is rough, for the attachment of ligaments. 



The anterior surface, of a somewhat triangular form, is smooth, concavo-convex, 

 and articulates with the cuboid. It is surmounted, on its outer side, by a rough 

 prominence, which forms an important guide to the surgeon in the performance 

 of Chopart's operation. 



The posterior surface is rough, prominent, convex, and wider below than above. 

 Its lower part is rough, for the attachment of the tendo Achillis ; its upper part 

 smooth, coated with cartilage, and corresponds to a bursa which separates this 

 tendon from the bone. 



Articulations. With two bones : the astragalus and cuboid. 



Attachment of Muscles. Part of the Tibialis posticus, the tendo Achillis, Plan- 

 taris, Abductor pollicis, Abductor minimi digiti, Flexor brevis digitorum, Flexor 

 accessorius, and Extensor brevis digitorum. 



