BONES, MUSCLES, EXERuISE, AND REST. 281 



we find some men are very strong in some of their limbs, 

 and weak in others ; and these strong and weak parts differ 

 according to the habits and employments of the people. A 

 sailor uses his arms more than his legs. He pulls ropes, 

 and lifts the anchor, but has little opportunity of walking. 

 Consequently, the muscles of his arms and shoulders are/ 

 large, strong, and hard, while his legs are smaller and 

 weaker than those of other men. The blacksmith uses the 

 same muscles, and has the same muscular development of 

 his arms, and comparative weakness of his legs. The pe- 

 destrian and the dancer have large and strong muscles of 

 the lower limbs, while their arms are comparatively small 

 and feeble. 



A similar difference of muscular development is manifested 

 in the porter. His employment calls fof'the exercise of the 

 muscles of the back more than that of other men. He car- 

 ries his burdens on his head or his shoulders. It is necessary, 

 therefore, for him to keep his spine erect by the constant and 

 vigorous action of the muscles of that region. They, there- 

 fore, are used more, and grow larger and stronger, than the 

 spinal muscles of men who are otherwise occupied. 



This law of special muscular development from special use 

 affects the lower animals as well as men. Hence we find in 

 them a growth and an increase of power in the muscles that 

 are used, and a comparative smallness and weakness in those 

 that are not used. The wild birds use their wings mostly for 

 locomotion. They sustain themselves very long in the air, 

 and fly great distances. They have, therefore, very large and 

 strong muscles on their breasts and wings. And, as they 

 walk but little, the muscles of their legs are comparatively 

 small and weak. 



But the domestic fowls seldom fly; they use their legs 

 mostly for locomotion. They have, therefore, much smaller 

 muscles on their breasts and wings, and much larger on their 

 legs. This difference is, perhaps, the most remarkably seen 

 in the wild and domestic turkeys and geese. 

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