BONES, MUSCLES, EXERCISE AND REST. 241 



PART VI. 



BONES, MUSCLES, EXERCISE AND REST. 



CHAPTER I. 



Bones, Composition of. Flexible in Childhood. Brittle in Old 

 Age. Strongest in Middle Life. Supplied with Blood- Vessels 

 and Nerves. Subject to Growth and Decay .- Grow strong by 

 Use. Should be used cautiously in Childhood. Rickets. 



583. The bones are hard, stiff, and very strong. They 

 are externally solid, Jbut are somewhat-hollow within. They 

 are composed of _such materials, that, without being heavy, 

 they are very firm, and formed in such a manner, that, 

 without being large or clumsy, they are very strong. The 

 composition of the bones is twofold the earthy and the 

 animal. The earthy part of the^bones is lime, or rather a 

 phosphate of lime^ This^gives them solidity and firmness. 

 The animal part is composed of gelatine, which is a sub- 

 stance similar to, glue. This gives the bonss their strength 

 and life. Either of these alone would make imperfect and 

 weak bones. 



584. When these two- elements the Ijme and the gela- 

 tine are united in due .proportions, the bones are very 

 strong, and will bear very Heavy shocks ; but, if either is 

 deficient, the body is not supported. When the lime is de- 

 ficient, the bones will bend ; and, .when the gelatine is defi- 

 cient, they will break. We see these different conditions 

 of the bones in the different periods of life. In early in- 

 fancy, the gelatine predominates, and the bones are soft and 

 yielding. They are then easily bent, but not easily broken. 



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