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 ; 
which is seldom found as an ingredient of 
Tocks or soils, except where it has been derived from animal 
‘Temains. 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 muriatie acid much 
diluted with water, the compounds of lime are entirely 
‘Temoved from it, and the organic substance remains ; 
‘i latter is now quite flexible, and almost transparent, so 
aA, 
Ey 
_that the distribution of its vessels (if they have been pre- 
“Viously injected with-colouring« matter) may be distinctly 
seen. On thé 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 
? torn into layers of an indistinctly-fibrous matting. In fact, 
