EFFECTS OF GROWTH. 



51. Growth produces in the species a somewhat remarkable 

 change in the mechanical qualities of the bones. This important 

 part of our organism consists of three constituents, fibre, cartilage, 

 and the earthy matter already mentioned called phosphate of lime. 

 From the fibre they derive their toughness ; from the cartilage 

 their elasticity, and from the lime their hardness and firmness. 

 Nothing can be more admirable in the economy of our body than 

 the manner in which the proportion of these constituents adapts 

 itself to the habitudes of age. The helpless infant, exposed by a 

 thousand incidents to external shocks, has bones, the chief consti- 

 tuents of which being gristly and cartilaginous, are yielding and 

 elastic, and incur little danger of fracture. Those of the youth, 

 whose augmented weight and increased activity demand greater 

 strength, have a larger proportion of the calcareous and fibrous 

 elements, but still enough of the cartilaginous to confer upon the 

 solid framework of his body the greatest firmness, toughness, and 

 elasticity." As age advances, prudence and tranquil habits increas- 

 ing, as well as the weight which the bones have to sustain, the 

 proportion of the calcareous constituent increases, giving the requi- 

 site hardness and strength, but diminishing the toughness and 

 elasticity. 



While the bones thus change their mechanical qualities as age 

 advances, they diminish in number, the frame consequently having 

 fewer joints and less flexibility. The bones of a child, whose 

 habits require greater bodily pliability, are more numerous than 

 those of an adult, several of the articulations becoming ossified 

 between infancy and maturity. In like manner, the bones at 

 maturity are more numerous than in advanced age, the same pro- 

 gressive ossification of the joints being continued. 



It has been ascertained by anatomists that, on attaining the 

 adult state, the number of bones constituting the framework of 

 the human body is 198 ; of which 52 belong to the trunk, 22 to 

 the head, 64 to the arms, and 60 to the legs. 



52. This wonderful solid structure is moved by a mechanical 

 apparatus, consisting of about 400 muscles, each of which is 

 attached at its extremities to two points of the body, more or less 

 distant from each other, which it has the power of drawing 

 towards each other by a contractile property peculiar to it. These 

 muscles, however, being passive pieces of mechanism, are moved 

 as already mentioned by the nerves, while the nerves themselves 

 are moved by the will, and here the material mechanism ends, 

 and the intellectual or the spiritual begins. 



As age advances, the organs lose their suppleness and elasticity; 

 the weight of the body undergoes a sensible diminution ; the 

 powers of digestion and assimilation are gradually impaired ; the 



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