50 GENERAL ANATOMICAL CHARACTERS 



One or more may be in a rudimentary condition, or altogether 

 suppressed. If one is absent, it is most commonly the first. 

 Excepting the Cetacea, no mammals have more than three phalanges 

 to each digit, but they may occasionally have fewer by suppression 

 or ankylosis. The first or radial digit is an exception to the usual 

 rule, one of its parts being constantly absent, since, while each of the 

 other digits has commonly a metacarpal and three phalanges, it has 

 only three bones altogether ; whether the missing one is a meta- 

 carpal or one of the phalanges is a subject which has occasioned 

 much discussion, and has not yet been satisfactorily decided. The 

 terminal phalanges of the digits are usually specially modified to 

 support the nail, claw, or hoof, and are called " ungual phalanges." 

 In walking, some mammals (as the Bears) apply the whole of the 

 lower surface of the carpus, metacarpus, and phalanges to the 

 ground ; to these the term " plantigrade " is applied. Many others 

 (as nearly all the existing Ungulata) only rest on the last one or two 

 phalanges of the toes, the first phalanx and the metacarpals being 

 vertical and in a line with the fore-arm. These are called " digiti- 

 grade." Intermediate conditions exist between these two forms, to 

 which the terms " phalangigrade " (as the Camel) and " subplanti- 

 grade " (as in most Carnivora), are applied. When the weight is 

 borne entirely on the distal surface of the ungual phalanx, and the 

 horny structures growing around it, as in the Horse, the mode of 

 progression is called " unguligiade." 



In the Chiroptera the digits are enormously elongated, and 

 support a cutaneous expansion constituting the organ of flight. In 

 the Cetacea the manus is formed into a paddle, being covered by 

 continuous integument, which conceals all trace of division into 

 separate digits, and shows no sign of nails or claws. In the Sloths 

 the manus is long and very narrow, habitually curved, and terminat- 

 ing in two or three pointed curved claws in close apposition with 

 each other, and incapable, in fact, of being divaricated ; so that it is 

 reduced to the condition of a hook, by which the animal suspends 

 itself to the boughs of the trees among which it lives. These are 

 only examples of the endless modifications to which the distal 

 extremity of the limb is subjected in adaptation to the various 

 purposes to which it is applied. 



Posterior Limb. — The posterior limb is constructed upon a plan 

 very similar to that of the anterior extremity. It consists of a 

 pelvic girdle and three segments belonging to the limb proper, viz. 

 the thigh, the leg, and the foot or pes (Fig. 15). 



Pelvic Girdle. — The pelvic girdle is present in some form in all 

 mammals, though in the Cetacea and the Sirenia it is in an exceed- 

 ingly rudimentary condition. In all mammals except those be- 

 longing to the two orders just named, each lateral half of the pelvic 

 girdle consists essentially, like the corresponding part of the anterior 



