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comparative facility with which quadrupeds keep 

 their heads above water, that they in general 

 swim better than man ; the legs in them being 

 the heaviest part, while in man the head is so. 

 Very few quadrupeds are capable of moving 

 through the air the bat, the flying squirrel, and 

 some species of lemar being among these ; and 

 this they effect, not like the flying fishes and 

 birds, by their anterior extremities alone, nor 

 like the flying lizard, by their ribs, but by wing- 

 like membranes extended between their anterior 

 and posterior extremities, the motions of both 

 which are requisite to call them into action. 

 Quadrupeds, in general, use their upper limbs 

 only in conjunction with their lower in the act 

 of progression, but some few, as squirrels, apes, 

 &c., use them also, in the same manner as we 

 use our arms ; and when such is the case, they 

 are furnished, like man, with a collar-bone, which 

 they are in general destitute of; the two bones 

 of their fore arm, also, are moveable upon each 

 other ; whereas they are otherwise consolidated 

 together. In standing, they use in general all 

 the four legs ; and, as the centre of gravity is 

 thus preserved without any effort, they easily 

 sleep in this posture ; some few, however, as the 

 kangaroo, the jerboa, &c., rest on the hinder legs 

 alone; the centre of gravity falling, in them, 

 almost perpendicularly : but such are accus- 

 tomed to use their strong tails almost like a third 

 leg, not only in steadying themselves, but in 

 assisting them in leaping. The chief muscles 



