178 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



buried are often found in this condition ; their form and position being 

 retained until they are exposed to the air, or are a little shaken, when 

 they crumble to dust. The proportion of the earthy matter of Bones 

 to the animal basis may be differently stated, according as we include, 

 in our estimate of the latter, the contents of the medullary cavity, the 

 Haversian canals, and the cancelli, or confine ourselves to that portion 

 only of the animal matter which is united with the calcareous element 

 in the proper osseous tissue. According to the recent experiments of 

 Dr. Stark,* the relative amount of the two elements, in the latter 

 estimate, is subject to very little variation, either in the different classes 

 of animals, or in the same species at different ages, the animal matter 

 composing about one-third, or 33 j' per cent., and the mineral matter 

 two-thirds, or 66f per cent. The degree of hardness of bone does not 

 altogether depend, therefore, on the amount of earthy matter they may 

 contain ; for the flexible, semi-transparent, easily-divided bones of Fish 

 contain as large an amount of animal matter, as the ivory-like leg-bones 

 of the Deer or Sheep. The usual analyses of Bone, however, have 

 been made upon the former kind of estimate : and they show that the 

 proportion of the earthy matter to the whole of the animal substance 

 contained in bone varies much in different animals, in the same animal 

 in different ages, and even in different parts of the same skeleton. The 

 reason of this will be apparent, when the history of the growth of Bone 

 has been explained ; since there is a gradual filling-up of all the cavities 

 at first occupied by fat-cells, vessels, &c., which does not cease with 

 adult age, but which continues during the whole of life. In this manner 

 the bones of old persons acquire a high degree of solidity, but they 

 become brittle in proportion to their hardness. From the same cause, 

 the more solid bones contain a larger proportion of bone-earth than 

 those of a spongy or cancellated texture ; the temporal bone, for exam- 

 ple, containing 63J per cent., whilst the scapula possesses only 54 per 

 cent. In the former of these bones, the proportion is nearly the same 

 as that which exists in pure osseous tissue, the amount of the remaining 

 tissues wliich it includes being very small, on account of the solidity of 

 the bone; but the latter contains in its cancelli a large quantity of 

 blood-vessels, fat-cells, &c., which swell the proportion of the animal 

 matter from 33J to 46 per cent. 



299. The Lime of bones is for the most part in a state of Phosphate, 

 especially among the higher animals ; the remainder is a Carbonate. 

 In Human bones, the proportion of the latter seems to be about one- 

 sixth or. one-seventh of the whole amount of bone-earth. In the bones 

 of the lower animals, however, the proportion of Carbonate is greater ; 

 and it is curious that in callus, exostosis, and other irregular osseous 

 formations in the higher animals, the proportion of the Carbonate 

 should be much greater than in the sound bone. In caries, however, 

 the proportion of the Carbonate is less than usual. The composition 

 of the Phosphate of Lime in Bones, is somewhat peculiar ; eight pro- 

 portions of the base being united with three of the acid. According to 

 Professor Graham, it is to be regarded as a compound of two tribasic 



* Edinburgh Medical and Surgical Journal, April, 1845. 



