This is the most marked differentiation of the connective tissue group. The intercellular substance is characterized by containing a great quantity of earthy or inorganic matter (65 %), which gives the bone its enormous strength. The cells of the tissue are enclosed in little cavities called lacuna, which are related by minute canaliculi to each other. The intercellular substance is everywhere traversed by the processes of the cells lying in the little canals which connect the lacunae, and thus the adequate nutrition of the tissue is secured. Chemically, bone tissue consists of about -

Parts.

Calcium Phosphate ..........................................

53

Calcium Carbonate....................................................

11

Magnesium phosphate, calcium fluoride and soda salts, ..........

1

Gelatin yielding animal matter ........................................

33

In the formation of bone from fibrous or cartilaginous tissue the original intercellular substance disappears, and a set of cells with new formative powers come upon the field (Fig. 34). These new cells (osteoblasts) cover the growing surface of the bone and secrete and lay down in layers a new kind of intercellular substance, which is the bone matrix. Here and there, at wonderfully regular intervals, an osteoblast ceases to secrete the calcareous intercellular substance, while its neighbors continue formative activity. Consequently, this osteoblast, or as it may now be called young bone cell, becomes surrounded by calcareous intercellular substance, and is permanently lodged in the bone tissue.