54 THE CONNECTIVE TISSUES [CH. V. 



white fibro-cartilage it does. In yellow fibro-cartilage the matrix is 

 pervaded by a deposit of elastin, which results in the formation of a 

 network of elastic fibres. 



Bone. 



Bone contains nearly 50 per cent, of water ; the solid material is 

 composed of earthy and animal matter in the proportion of about 67 

 per cent, of the former to 33 per cent, of the latter. The earthy 

 matter is composed chiefly of calcium phosphate, but besides this, 

 there is a small quantity (about 11 of the 67 per cent.) of calcium 

 carbonate, calcium fluoride, and magnesium phosphate. 



The animal matter is chiefly collagen, which is converted into 

 gelatin by boiling. 



The animal and earthy constituents of bone are so intimately 

 blended and incorporated the one with the other that it is only by 

 severe measures, as for instance by a white heat in one case and by 

 the action of -concentrated acids in the other, that they can be 

 separated. Their close union too is further shown by the fact that 

 when by acids the earthy matter is dissolved out, or on the other 

 hand when the animal part is burnt out, the shape of the bone is 

 alike preserved. 



The proportion between these two constituents of bone varies 

 slightly in different bones in the same individual and in the same 

 bone at different ages. 



To the naked eye there appear two kinds of structure in different 

 bones, and in different parts of the same bone, namely, the dense or 

 compact, and the spongy or cancellous tissue. Thus, in making a 

 longitudinal section of a long bone, as the humerus or femur, the 

 articular extremities are found capped on their surface by a thin 

 shell of compact bone, while their interior is made up of the spongy 

 or cancellous tissue. The shaft, on the other hand, is formed almost 

 entirely of a thick layer of the compact bone, and this surrounds a 

 central canal, the medullary cavity so called from its containing the 

 medulla or marrow. 



In the flat bones, as the parietal bone or the scapula, the can- 

 cellous structure (diploe) lies between two layers of the compact 

 tissue, and in the short and irregular bones, as those of the carpus 

 and tarsus, the cancellous tissue fills the interior, while a thin shell 

 of compact bone forms the outside. 



Marrow. There are two distinct varieties of marrow the red 

 and yellow. 



Red marrow is the connective tissue which occupies the spaces in 

 the cancellous tissue ; it is highly vascular, and thus maintains the 

 nutrition of the spongy bone, the interstices of which it fills. It 

 contains a few fat-cells and a large number of marrow-cells. The 



