OS CALCIS. 251 



obliquely forwards and outwards, and is rough for the attachment of the inter- 

 osseous ligament connecting the astragalus and os calcis. Of the 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 back- 

 wards. The internal articular surface is supported on a projecting process of 

 bone called the lesser process of the calcaneum (sustentaculum tali); it is also 

 oblong, concave longitudinally, and sometimes subdivided into two parts, which 

 differ in size and shape. More anteriorly is seen the upper surface of the 

 greater process, marked by a rough depression for the attachment of numerous 

 ligaments, and the origin of 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 

 attachment to 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 the outer head of the Flexor 

 Accessorius muscle ; 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. Above the tubercle is a broad smooth surface, giving attach- 

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

 ment ; and in front of the tubercle a narrow surface marked by two oblique 

 grooves, separated by an elevated ridge : the superior groove transmits the ten- 

 don 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 and 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 bone, the lesser pro- 

 cess, which projects horizontally inwards from its upper and fore part, and to 

 which a slip of the tendon of the Tibialis Posticus is attached. 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-con- 

 vex, 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 per- 

 formance of Chopart's amputation. 



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

 above. Its lower part is rough, for the attachment of the tendo Achillis, and 

 of the Plantaris muscle ; its upper part is smooth, coated with cartilage, and 

 corresponds to a bursa which separates that tendon from the bone. 



Articulations. With two bones : the astragalus and cuboid. 



Attachment of Muscles. Part of the Tibialis Posticus, the tendo Achillis, 

 Plantaris, Abductor Pollicis, Abductor Minimi Digiti, Flexor Brevis Digito- 

 rum, Flexor Accessorius, and Extensor Brevis Digitorum. 



THE CUBOID. 



The Cuboid Bone is placed on the outer side of the foot, in front of the os 

 calcis, and behind the fourth and fifth metatarsal bones. It is of a pyramidal 

 shape, its base being directed upwards and inwards, its apex downwards and 



