COMPOSITION OF BONE. 63 



supplied by the blood-vessels, and to transmit it to the 

 outer circles, or those furthest removed from those vessels ; 

 and in this manner, a much more active nutrition takes place 

 in bone than that which is performed in cartilage. It has 

 been proved by various experiments, that the substance of 

 bone is undergoing continual change ; and it is owing to the 

 comparative activity of its nutritive processes, that bone is so 

 readily and perfectly repaired, when it has been broken by 

 violence or has been injured by disease. 



51. But the peculiarity of Bone consists, not so much in 

 this remarkable arrangement of its organic structure, as in its 

 solidity and firmness. This is given to it by the union of 

 a large quantity of mineral matter with the organic substance 

 of its tissue. The mineral matter of bones consists almost 

 entirely of two compounds of Lime; the carbonate, with 

 which we are familiar in the form of limestone and chalk ; 

 and the phosphate, which is seldom found as an ingredient of 

 rocks or soils, except where it has been derived from animal 

 remains. The latter greatly predominates, at least in the 

 bones of the higher animals. We. may easily separate the 

 animal and the mineral portions of the bony tissue. If we 

 soak a small bone for some time in muriatic acid much 

 diluted with water, the compounds of lime are entirely 

 removed from it, and -the organic substance remains; 

 the latter is now quite flexible, and almost transparent, so 

 that the distribution of its vessels (if they have been pre- 

 viously injected with colouring matter) may be distinctly 

 seen. On the other hand, if we subject a bone to strong 

 heat, the animal portion will be burnt out, and the earthy 

 matter will remain. The form of the bone will be still 

 retained ; but the cohesion between the earthy particles is so 

 slight, that the least touch will break them asunder. Thus 

 we see that the hardness of bone, or power of* resisting pres- 

 sure, is given by the earthy matter; whilst its tenacity, or 

 power of holding together, depends upon the animal portion. 

 Although the animal substance which remains after the solu- 

 tion of the mineral matter, has been commonly described as 

 Cartilage, yet it is not so in reality ; for it consists not of 

 chondrin, but of gelatin ; and instead of being made up of an 

 aggregation of cells united by an intervening substance, it may 

 be torn into layers of an indistinctly-fibrous matting. In fact, 



