170 COMPARATIVE ANATOMY 



recognised as a typical element in all pentadactyle Mammals ; but 

 as a rule it later becomes fused with one, or even with two, of the 

 neighbouring carpals generally with the radiale, less frequently 

 with carpal e 2 or 3. Occasionally indications of a second centrale 

 are seen, which usually fuses with the intermedium (Homo). 

 Similar fusion and shifting in relative position may also occur 

 in other carpals and tarsals (e.g. radiale and intermedium). 

 The " pisiform " corresponds to an additional ulnar ray, and not 

 to a sesamoid. 



In the tarsus the centrale (navicular) is retained, and is usually 

 situated on the inner (tibial) border : it may be primarily double. 

 The astragalus possibly corresponds to the tibiale and intermedium, 

 and the calcaneum to the fibulare, while the ciiboid represents 

 tarsalia 4 and 5. 



Traces of a " prepollex " and " prehallux " are present in all 

 pentadacyle Mammals, especially in lower forms, in which they may 

 each consist of two or more elements : in the higher Mammals 

 there is never more than one such bone, which usually becomes 

 fused with its neighbours. 1 



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

 number may be reduced, 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. 134). 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. 

 The short humerus is enclosed in the body-wall in Toothed 

 Whales, which possess five digits, the fourth of which commonly 

 bifurcates in the embryo ; Whalebone-Whales possess only four 

 digits. 



It is interesting to trace the reduction which has taken place 

 in the feet of the true Ungulates in the course of time. This 

 order has been undoubtedly derived from that of the Carnivora, 

 the fossil Condylarthra from the American Eocene and the tri- 

 tubercular Creodonta from the Cretaceous forming connecting 

 links between the two. In the Eocene, the Ungulata vera 

 diverged into two groups, the Perissodactyla (Tapir, Rhinoceros, 

 Horse) and Artiodactyla (Pigs, Hippopotami, Ruminants). In 

 Fig. 134 sketches of the stages in the phylogenetic development 

 of the fore-foot of the Horse are given, showing how it has been 

 gradually derived from a tetradactyle form : the embryo passes 

 through these stages in the course <>f its development. While in 

 this case the third digit becomes greatly enlarged relatively 



1 Different views have been expressed as to the morphological nature of the 

 prepollex and prehallux, which m consequence of functional adaptation may 

 undergo further development in some Mammals (e.g. Talpa). It is not possible 

 in all cases to make a satisfactory comparison between individual elements of 

 the carpus and tarsus, or tohomologise the so-called "accessory elements." 



