8 



OSTEOLOGY. 



The animal and earthy proportions do not vary in the true bone 

 tissue, but there is a gradual filling up of the cavities originally 

 occupied by fat cells, thus condensing the bone. The earthy 

 ingredients consist chiefly of carbonate and phosphate of lime, 

 the animal matter of cartilage and connective tissue, vessels, 

 membranes lining the cancellated structure and medullary 

 cavities, and a quantity of fat. The following is an average 

 analysis of osseous tissue by Von Bibra : 



In adult bones, as above stated, the proportion of animal 

 matter is about a third, varying in different animals, and in 

 different bones of the same animal. The bones of the young may 

 have too great a proportion of animal matter, when they give way 

 under the superincumbent weight, as in rachitis, when, ossifica- 

 tion advancing, the animal becomes a cripple. The degree of 

 hardness varies not only with the age, but also with the class of 

 the animal, the bones of birds being white, hard, and brittle, 

 especially those of the wings and legs ; the bones of the chamois 

 and its congeners are aJ.?o very hard and brittle, whereas fish 

 bones are soft and flexible. The bones of some fishes contain a 

 very large proportion of animal matter, and these are known as 

 the cartilaginous fishes. In different parts of the same skeleton 

 they vary in hardness, the petrosal bone being the hardest one in 

 the body; the shank bones are also very hard and dense, while 

 the ribs are soft and flexible. The leg bones of a thorough- 

 bred horse are more compact than those of a heavy cart-horse ; 



