OSSEOUS SYSTEM OF MAMMALIA. 87 



their hoof-like claws with great effect in demolishing ant-hills, built of 

 indurated earth, in order to obtain the insects on which they feed : hence 

 the bones of the anterior limbs indicate, in their solidity, and the 

 abrupt spines and projections covering their surface, the vast force of the 

 muscles which act upon them. It might, indeed, have been suspected, 

 that, in these animals, some trace, at least, of clavicles would have 

 appeared : such, however, is not the case. In the Sloth this important 

 bone, the clavicle, is articulated not only to the acromion of the scapula, 

 which is remarkably elongated, but to the coracoid process also ; these 

 two processes absolutely meeting at their points, so as to enter mutually 

 into union with that bone. The scapula is large ; the humerus is long 

 and slender, as are also the bones of the fore-arm. The hand is a most 

 extraordinary piece of mechanism : it is a hook (trebled in one species, 

 and doubled in another) ; a hook for clinging to the branches of the 

 forest, on the underside of which the Sloth habitually lives, suspended 

 with the back downward. In this manner it proceeds (like a fly on the 

 ceiling) with great rapidity. To have given the Sloth a grasping hand 

 would have been futile ; because no voluntary muscles could endure a 

 perpetuity of action, which must necessarily be continued without inter- 

 ruption, as well during the repose, as during the exertions, or progression, 

 of the animal. To avoid this, the hand (and the same observation applies 

 to the hinder extremities) is so constituted, as to act mechanically as a 

 hook. Externally, the skin envelops every part, except the enormous 

 claws ; and these claws, of an arched figure, have a tendency inward, 

 viz., towards the palm or sole ; insomuch that, when the animal is removed 

 from its branch, and placed on the ground, where it is absolutely helpless, 

 these claws fold down close upon the palm, requiring the action of the 

 extensor muscles to restore them to their usual state, which is one of 

 flexion, at right angles with the palm. Now, it is well to observe, that the 

 inward contraction of these claws is not the result of muscular action, but 

 occasioned by the simple elastic action of ligaments destined to effect 

 it ; whilst to unclose them requires voluntary muscular exertion. The 

 wisdom of this curious and beautiful arrangement is evident. The Sloth 

 lives suspended, and the flexion of its hooks, and the security of its hold, 

 are effected by the elasticity of these guardian ligaments, which, like spiral 

 springs, are ever in operation, counteracting the tendency of the body's 

 weight to unclose them. Fig. 89 represents the arm and hand of the Sloth 

 (Ai) ; in this species the carpus, d, is composed of six bones ;* four in the 

 first row, two in the second : the scaphoid is the largest. The bones of the 

 metacarpus, e, are all consolidated together at their base, and, with the 

 anterior carpal bones ; they consist of three large bones, on which rest 



* In the carpus of the Unau there are seven bones. 



