LIMBS 



133 



radiale {e.g., the Gorilla, the Chimpanzee, and Man, though 

 it may persist in the human subject throughout life or may 

 fuse with carpale 2 or '6). In the tarsus the centrale (navicular) 

 remains distinct, and usually lies on the inner border of the foot. 



So much difference of opinion exists with regard to the homologies of the 

 bones of the carpus and tarsus in Mammals, that it is not possible at present 

 to give a satisfactory account of them, or of the additional elements which are 

 often present in the embryo and disappear during- development. Thus the 

 pisiform may be a true sesamoid, or may represent an additional ulnar ray, 

 and the calcaneum may or may not be the complete serial homologue of the 

 pisiform. Elements occur occasionally in the carpus and tarsus which are 

 supposed to represent additional radial and tibial rays respectively — the 

 so-caUed ^epoKex snxdi prehallux (Fig. 113). 



There are typically five complete digits on each foot, but tliis 

 number may be reduced to four, three, or even one (Figs. 114 and 



IT. B 



Cuh- 



FiG. 113. — A, Cakpus, and B, Skeleton of the Foot of Max. (The rudiments 

 of the so-called prepoUex and prehallux (itf) are represented diagram- 

 matically. 



TJ, ulna ; i?, radius ; r, radiale ; i, intermedium ; )(, ulnare ; P, pisiform ; ce, 

 centrale, fused with the radiale ; ce"-, second centrale, forming the head 

 of tarsale 3 ; 1-5, the carpalia and tarsalia, 4 and 5 being united to form 

 the unciform and cuboid respectively ; Cii I-III, the first to third tarsalia ; 

 c, centrale tarsi (navicular) ; it, intermedio-tibiale = astragalus {As) ; 

 f + p, calcaneum (= fibulare and pisiform tarsi?); I-V, the metacarpals 

 and metatarsals. 



115), the disappearance taking place in the following order — 

 1, 5, 2, 4 : thus in the horse the third is the only complete digit 

 remaining (Fig. 115). The number of phalanges is similar in both 

 hand and foot : in the first digit there are only two, while in the 

 others there are three. An exception to this rule is seen in 

 Cetacea, in which the phalanges are numerous, as in Ichthyosaurus 

 and Plesiosaurus amongst Reptiles. 



It is interesting to trace the reduction which has taken place in the feet 

 of Ungulates in the course of time. Fig. 115 represents successive stages in the 



