738 MAMMALIAN ANATOMY 



THE TARSUS. 



General Description. The proximal division of the foot is 

 constituted, like the carpus, of seven small bones closely united ; it 

 differs, however, from the carpus in important details. In the first 

 place, it is much longer, forming a third of the entire foot and being 

 a third as long as the tibia, whereas the carpus forms but an eighth or 

 ninth of the hand and is only a twelfth as long as the ulna. In the" 

 second place, its articulation with the lower ends of the leg bones is 

 not by means of a well-defined proximal surface, but, because of the 

 backward development of the heel bone, is effected by a large articular 

 swelling on the dorsal surface, a condition which greatly limits the 

 amount of backward and forward motion, or flexion, of the foot upon 

 the leg. 



The proximal end of the tarsus is formed by the narrow end of 

 the heel bone, from which the lateral margins diverge to the middle of 

 the tarsus, whence they are parallel for the rest of their extent. The 

 outline of the tarsus as seen from the dorsal aspect may therefore be 

 said to be formed of a proximal triangle, whereof the base is applied 

 to a distal square. 



The tarsus has four surfaces, the dorsal, the plantar, the proximal, 

 and the distal, and two borders, the tibial and the fibular. 



The dorsal surface is narrow, and little more than a border in the 

 proximal two-sevenths ; in the next two-sevenths it is wider, and 

 swollen into a prominent, spool-like articular eminence ; and for the 

 remaining three-sevenths it is quadrate, smooth, and convex from side 

 to side, with the maximum convexity on the tibial side of the middle 

 line. The dorsal surface forms part of the instep. 



The plantar surface has the general outline of the dorsal surface, 

 but is flatter. The proximal half is transversely convex and smooth ; 

 the distal half is irregularly depressed and elevated for ligamentous 

 attachment. The plantar surface forms part of the sole of the foot. 



The proximal surface is the circular end of the heel bone. 



The distal surface is transversely oval, or, more accurately, slightly 

 crescentic, with the arcuate border on the dorsal side and the straight, 

 or slightly emarginate, border on the plantar side. It is not perfectly 

 flat, but elevated and depressed, owing to the different lengths of the 

 constituent tarsal bones. It is highest in the middle and at the tibial 

 end, and most depressed between the middle and the tibial end. 



