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PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



It is evident that with a hollow skeleton and muscles 

 within, only hinge joints can be formed. A ball and 

 socket is impossible. How, then, is universal motion 

 effected ? This is done by two hinge joints moving in 

 planes at right angles to one another, as in the diagram, 

 Fig. 149. If we examine the leg of any arthropod, say 



Fig. 150. — Four joints of the limb of a crustacean cut so as to show the 

 muscles (m m) and their attachments. 



a lobster, we shall find that the consecutive joints are 

 hinged alternately in planes at right angles to one 

 another (Fig. 150). 



Now of these two different modes of relation of 

 skeleton and muscle, which is the best ? We have al- 

 ready seen the intense locomotive activity of insects. 

 Many writers hastily conclude that the nervous and 

 muscular activity of insects is far greater than that of 

 vertebrates, or else that their mechanism is superior. 

 This is probably a mistake. The superior locomotive 

 activity of insects is simply the result of small size. It is 

 evident that, other things being equal, the contractile 

 power of a muscle varies as its cross section — i. e., as the 

 square of its diameter. But the weight to be moved— 

 i. e., the weight of the body — varies as the cube of the 

 diameter. Therefore, as the size of the animal in- 

 creases, its weight increases faster than its muscular 

 power. Therefore more and more of the whole energy 

 is used up for support of weight, and less and less is 

 left over for locomotion, until, if the animal is large 

 enough, the whole power is used up for support, and 

 none is left over for locomotion. There is therefore a 



