THE CALCANEUM. 



421 



notch in the free border just at the end of the sustentaculum. Occasionally the 

 facet in front of the interruption is rudimentary or wanting, in which case, instead 

 of articular cartilage, merely synovial membrane is beneath the head of the astraga- 

 lus. In 200 feet we have found the facet single in 95, divided in 94, and in 11 the 

 front was wanting. The two chief facets (counting the anterior as one, even if sub- 

 divided) are separated by a deep groove for the interosseous ligament to the astrag- 

 alus. This gutter broadens in front into a rough depression, the sinus tarsi, for 

 ligaments. At its outer part there is a tubercle for the origin of the extensor brevis 

 digitorum. The anterior surface, turned somewhat inward, is wholly articular 

 for the cuboid. It is three-sided with rounded angles. The longest diameter is 

 from above downward and outward, nearly parallel with the inner border. The 

 upper border is straight or convex, overhanging the joint at the inner side. The 

 outer border slants a little inward as it descends. The surface is concave from 

 above downward and convex transversely. Both these curves are most marked at 



Fig. 432. 



Cuboid 



Astragalus 



Sustentaculum tali, 



Interosseous groove 



Internal tubercle 



Sinus tarsi 



Peroneal spine 



Astragalus 



For bursa 



Tendo Achillis 

 Right calcaneum from above. 



the upper inner angle, where they form almost a groove for the plantar process of 

 the cuboid. The general effect is of a screw surface twisting upward and inward. 

 The calcaneum articulates with two bones, the cuboid and the astragalus, and excep- 

 tionally with the scaphoid, to which it may be united by cartilage. 



Variations. — The hind end of the sustentaculum is very rarely a separate 

 piece : os sustentaculi propriimi. The inner edge of the front of the bone, which 

 normally comes very near to the scaphoid, may meet it. Sometimes the two bones 

 are fused. The calcaneiivi secundarium is a small ossicle rarely present on the 

 dorsum between the calcaneum, the cuboid, the scaphoid, and the head of the 

 astragalus. Fusion of the calcaneum and astragalus has been observed at the sus- 

 tentaculum. 



Structure. — The walls are thin, the cancellated tissue filling the bone, with a 

 tendency to the formation of large spaces at the middle. The architectural arrange- 

 ment is very clear in an antero-posterior section, which shows diverging plates from 



