16 STRUCTURE OF THE BONES. 



— The human bones m a recent adult subject, are of a dull 

 white color : they possess considerable elasticity, but little 

 flexibility, have the greatest specific gravity of any portion of 

 the human body, and are liable to be broken by accidents or 

 even by violent muscular efforts. Their texture is varied not 

 only in different parts of the skeleton, but in different parts of 

 the same bones. Thus in the long bones, the middle portion 

 or diaphysis is compact, or nearly solid, with a cavity in the 

 centre ; the extremities are cellular or spongy, with but a thin 

 coating of the compact matter ; and the central cavity, is occu- 

 pied by a long network formed of thin plates and fibres, called 

 the reticulated tissue of the bones. 



— In flat bones the external surfaces are composed of firm 

 plates of compact bone ; but the internal substance is cellular. 

 In some of these bones, the cellular tissue exists in such small 

 quantity, that the external compact layers almost touch, and the 

 bones become then diaphanous or translucent. 

 — The thick bones are formed almost entirely of the spongy or 

 cellular substance, which is surrounded by an extremely thin 

 shell of the compact bony matter, and are somewhat darker in 

 color than the long or the flat. 



— The osseous tissue thus presents three modifications of form ; 

 the compact, the reticular, and the cellular or spongy. The 

 compact, which is the densest and strongest, is placed upon the 

 outer surface of all the bones of the body ; it forms a covering 

 of greater or less thickness to all the flat and thick bones, and 

 adds to their strength, without much increasing their weight. 

 The long bones, which are narrowed down in the shaft, so as 

 to accommodate the muscles without destroying the symmetry 

 of the limbs, and require to be made of the strongest material, 

 have their shafts or bodies formed almost entirely of the com- 

 pact portion. The cellular or spongy is found, in a greater or 

 less degree, in every bone of the body ; in the extremities of 

 the long bones it is continuous, though indirectly, with the 

 reticulated tissue of the central or medullary canal. The 

 reticulated tissue has been considered only a modification of the 

 spongy, being formed of larger cells with a more open and 

 delicate texture. 



