136 LECTURE III. 



This little bone is analogous to the tail 

 of animals, which is moveable on the end 

 of the sacrum, and composed of many 

 pieces of various form in different animals. 

 I mention this merely as one of the proofs, 

 though it is far from the best, of the uni- 

 formity of the plan, and the diversity of 

 the means and ends we observe in the 

 construction of animals. The tail very 

 commonly serves to animals the purposes 

 of a switch, a balance, and a rudder. It 

 is however a hold-fast in such as possess 

 the Cauda prehensilis, whilst to the kan- 

 garoo it is a prop and a leaping-pole, and 

 to the beaver it serves as a trowel. 



The sacrum derives support from the 

 two side bones of the pelvis, by extensive 

 surfaces, which converge as the wedge di- 

 minishes. These bones form a kind of 

 circle, and are united with each other in 

 front, and thus is the bony circumference 

 of the pelvis completed. The back part 

 of the circle being continued from the sa- 

 crum forms an arch on which the weight of 

 the body is supported. No one can, I 



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