BODY TISSUES 299 



through the thoracic duct. Waste products not absorbed by the 

 capillaries also enter the venous blood stream for disposal. 



SUPPORTING TISSUES 



Bones 



Bones form the skeletal framework of the body and are character- 

 ized, chemically, by a high projjortion of mineral matter. In the 

 embryo, bones appear first as cartilaginous structures consisting of 

 cells embedded in a homogeneous intercellular substance, largely 

 l^rotein. The protein present in the largest amount is called collagen. 

 Other proteins, present in smaller amounts, are osseomucoid and 

 osseoalbuminoid. When ossification takes place, the intercellular 

 spaces fill with mineral salts of which a salt closely related to tricalcium 

 phosphate predominates. Although authorities are not in complete 

 agreement, evidence indicates that the mineral matter in bone 

 consists of a complex salt, the formula for which may be written 

 3Ca;^(P04)2-CaXo. The carbonate radical C03= usually occupies the 

 Xo position, but other radicals, such as 2F-, S04=, or 0=, may also 

 constitute this part of the molecule. Magnesium can replace calcium 

 in bone, but the amount of magnesium in bone appears to be rela- 

 tively constant. Fluorine is always present in normal bone in small 

 amounts. 



Bone marrow, the principal source of red blood cells, consists of 

 organic material containing a protein, called ossein, and relatively 

 large amounts of fatty materials. 



The normal mature bone is composed of nearly 50 per cent water, 

 20 to 25 per cent fat, 15 to 17 per cent ash, 7.5 to 10 per cent protein, 

 and 1 per cent citric acid. On a moisture-free, fat-free basis, 60 to 70 

 per cent of the bone is inorganic in nature with the following com- 

 position: 



Appreciable amounts of additional inorganic components may be 

 found in bone under special circumstances, such as lead in cases 



