THE SKELETON OF THE HIND-LIMB 57 



fossil representatives. The tibiale and intermedium are 

 fused to form the astragalus, while the fibulare remains 

 large and distinct as the calcaneum, thus reducing the 

 first row to two bones. The centrale remains as the 

 large scajghoid; tarsalia I., II., and III. persist as distinct 

 elements, the inner, middle and external cuneiform bones ; 

 while tarsalia IV. and V. fuse into the cuboid. This con- 

 stitutes the minimal mammalian reduction (and indeed 

 the minimal reduction of existing Reptiles), and as such 

 it is typical of the Primates (see Fig. 20). It is also seen 

 in the Insectivora and in some other orders, but it is 



FIG. 20. SKELETON OF THE HUMAN FOOT, TO SHOW THE FATE 

 OF THE PRIMITIVE ELEMENTS OF THE TARSUS. 



carried farther by other fusions and reductions in most 

 highly specialized quadrupedal Mammals. 



The five primitive digits remain in their elemental 

 development and relative proportions in the Primates, 

 as well as in the Insectivora and some other orders, and, 

 like the thumb, the big to^ has undergone a reduction by 

 the loss of one element. In most of the members of the 

 Primate stock the primitive formula of the digits is 

 retained, and the third or middle toe outstrips its neigh- 

 bours as does the middle finger. 



It may therefore be claimed for the typical Primate 

 hind-limb that almost as far as original simplicity is 

 retained in the Mammals it is here present in all the 

 skeletal elements. In mobility of the second segment 

 only is the Primate simplicity surpassed by the Mono- 

 tremes and by the arboreal Metatheria. 



