536 CHORDATA. 
renders the animal quite unfit for terrestrial locomotion 
(f- bat and whale), the horse being equally unfitted for 
climbing or burrowing. 
The pelvis is much smaller than in the terrestrial types, 
the comparison in this respect with its near ally, the ground 
sloth (Megatherium), being very striking. The hind-limb is 
similar in general characters to the fore-limb. The femur 
is long and slender; the tibia and fibula are of about equal 
size and allow the foot to face inwards. The foot, like the 
hand, is elongated and has the same condition of the digits, 
the second, third and fourth alone remaining. The tarsus 
shows the same fusion and reduction of elements as the 
carpus, all the bones but the astragalus becoming fused 
Fig. 370.—STOMACH OF SLOTH. 
Note the complex folds and the two-chambered condition. 
together in many individuals. As in the case of the meta- 
carpals, so here the three metatarsals are fused together into 
one bone. The fibula has lost its connection with the 
calcaneum, but articulates with the astragalus. 
At the base of the limbs the brachial arteries break up 
into networks of vessels, known as vefia mirabilia, an adap- 
tation apparently serving to overcome the effects of gravity 
upon the circulation of the limbs (see page 464). The 
mamme are pectoral, a position common amongst arboreal 
and zerial types. 
The stomach of the sloth, as in most herbivorous 
animals, is complex, consisting of at least two chambers, 
each of which has an appendix or caecum. 
