424 



HUMAN ANATOMY. 



the back of the tibia. A much smaller similar surface is found at the front, made 

 by the corresponding anterior ligament. The direction of the anterior border of the 

 articular surface is very uncertain. It usually projects forward at the outer end, the 

 rest being either transverse, posteriorly concave, or oblique. Just anterior to it is a 

 deep transverse hollow on the upper surface of the neck, which receives the edge of 

 the tibia in extreme dorsal flexion of the foot. The posterior border of the articular 

 surface is also of uncertain shape. Its inner end is usually somewhat farther back 

 than the outer. Behind it two rough tubercles project backward, slanting down to 



Fig. 437. 



Sustentaculum 



For scaphoid 



For 



sustentaculum 



For ligament 



Calcaiieum Astragalus 



Type of calcaneo-astragaloid joint with an undivided anterior articular facet on calcaneum. 



a posterior sharp edge. Between them is a deep groove for the tendon of the flexor 

 longus hallucis, running obliquely downward and inward. The outer tubercle, 

 which is much the larger, is sometimes separated by a suture from the rest of the 

 bone, and is then known as the os trigonum. The inner tubercle may be barely 

 distinguishable. This region behind the superior articular facet is sometimes de- 

 scribed as the posterior surface of the bone. The external surface of the body 

 shows the triangular facet for the outer malleolus, concave from above downward. 



Fig. 438. 



Synovial not cartilaginous 



Sustentaculum 



For scaphoid 



For internal calcaneo- 

 scaphoid ligament 



For sustentaculum 



Calcaneum 



Astragalus 



Type of calcaneo-astragaloid joint when anterior facet on calcaneum is not only divided but has front portion 



rudimentary. 



with the lower end projecting outward, plane or convex from before backward. 

 This is bounded before and behind by a rough strip, with a hollow at the upper 

 ends for the front and back bundles of the external ligament of the ankle. The 

 internal surface has at the top a narrow curved facet for the inner malleolus, with 

 a concave lower border, deepest in front and pointed behind. A part of the internal 

 lateral ligament is inserted into a hollow below it. The inferior surface of the 

 body presents a four-sided facet, concave in the line of its long axis, which is oblique, 

 corresponding to that of the greater surface on the top of the calcaneum. In front 

 of and parallel to this is a deep groove for the interosseous ligament, expanding at 

 the outer end into a triangular hollow on the under side of the neck. This is a 



