30 THE HUMAN BODY. 



they are flexible and have only a resistant power propor- 

 tionate to the movements and efforts of infancy; it is the 

 branch full of sap, but of which the woody portion is not 

 yet developed. In youth, as the muscular power augments, 

 the bones gradually become more solid; the extremities, at 

 first cartilaginous, become ossified; the epiphyses unite with 

 the body of the bone; and the articulating cartilages gain 

 more consistence. In the adult at last the bones are 

 complete. They are able to resist the muscular efforts of 

 maturity, and perform their functions perfectly, like all other 

 parts of the organism which are fully developed. But age 

 approaches, the strength decreases, nutrition falls off, the 

 bones become more solid, their power of resistance lessens, 

 the medullary canal enlarges, the proportion of calcareous 

 salts in the osseous substance augments, the bones are harder 

 but they are also more brittle. Thus, as each phenomenon 

 of life is linked with every other, we find the bones of a 

 child quickly and easily restored if broken; in the adult the 

 process may be longer, but is generally easy and complete; 

 in the old man the reunion of the fragments takes place 

 slowly, and sometimes cannot be effected at all. The deli- 

 cate twig, which afterwards became a vigorous branch, is 

 now dry and destined to speedy decomposition. 



Joints. The bones are attached to each other, either by 

 their extremities or their sides, in such a manner as to per- 

 mit freedom of motion to a greater or less extent between 

 the different parts of the skeleton and of the body. Held 

 together by a sort of cog-wheel system, by the fitting of a 

 projection into an appropriate cavity, or by juxtaposition, 

 they are maintained in connection either by the reciprocal 

 attachments of these projections, or by envelopes the 

 articular capsules and by ligaments, constant in their nature, 

 but varying in form and disposition, according to the dif- 

 ferent movements which they are designed to permit and 

 insure. 



This assemblage, this connection of the bones, is termed 

 a joint. Joints are classed according to the form of the 

 articulating surfaces, and according to the extent and variety 

 of the movements produced by them. The bones of the 



