LIMBS OF VERTEBRATES 



'241 



I 



X/s. 



\Y- 



three each) and consequently the first to leave the ground as 

 the hinder part of the foot is uplifted. The next to go is the 

 postaxial digit (No. V) from the opposite side, the next No. II 

 and the next No. IV. 



In some ungulates the long axis of the foot passes through 

 the middle digit (No. Ill) and in others between the third and 

 fourth digits. The true ungulates are 

 accordingly divided into two series, odd- 

 toed or perissodactyl and even-toed or 

 artiodactyl. 



To the former belong the tapir 

 (Fig. 96), with four digits in the fore foot 

 and three in the hind ; the rhinoceros, 

 with four or three digits in the fore foot 

 and three in the hind, and the horse 

 (Fig. 97), with only the middle (third) 

 digit remaining in both manus and pes, 

 but with vestiges of Nos. II and IV in 

 the functionless " splint-bones " which 

 lie alongside the greatly elongated meta- 

 podial (= metatarsal or metacarpal) of 

 No. III. 



In the artiodactyl series (Fig. 98) we 

 find the hippopotamus, with four well- 

 developed toes on each foot; the pig, 

 with four toes on each foot but the two 

 outer ones greatly reduced; the deer, 

 with the two outer digits still further 

 reduced ; the sheep and oxen, with the 

 second and fifth digits reduced to small 

 nodules of bone, and the camels and 

 llamas, in which all traces of digits other 

 than the third and fourth have completely disappeared. The 

 gradual reduction of the number of digits in the artiodactyl series is 

 accompanied by fusion of the two remaining metapodials to form 

 a single" cannon bone " (well shown in Fig. 98, deer and camel), 

 whereby the now greatly elongated and uplifted foot acquires 

 much greater strength and rigidity. 



It is very easy to see that all the different varieties of 

 perissodactyl and artiodactyl limbs are modifications of one 

 and the same primitive pentadactyl type. We must next 



B. R 



FIG. 97. Fore and hind 

 Feet of a Horse, show- 

 ing reduction to a single 

 functional Digit (No. 

 Ill) with Vestiges of 

 two others (II and IV) 

 in the form of splint 

 bones, X J-. (From 

 Lull, after Marsh.) 



