Section 69. Among the bony bars and spicules of spongy bone occurs the red marrow-- which must not be confused with the yellow marrow, the fatty substance in the central cavity of long bones. In this red marrow are numerous large colourless cells, which appear to form within their substance and then liberate red blood corpuscles. This occurs especially in the spongy bone within the ribs. Section 70. The matrix of bone differs from that of cartilage or of most other tissues in consisting chiefly of inorganic salts. The chief of these is calcium phosphate, with which much smaller quantities of calcium carbonate, and magnesium phosphate and carbonate occur. These inorganic salts can be removed by immersion of the bone in weak hydrochloric acid, and a flexible network of connecting tissue, Haversian vessels, bone corpuscles, and their processes remains. This is decalcified bone alluded to above. Section 71. In the very young rabbit, the limb bones, vertebral column, and many of the skull bones are simply plates and bars of cartilage; the future membrane bones, however are planned out in connective tissue. The development of the latter is simple, the connective tissue corpuscles functioning by a simple change of product as osteoblast. The development of the cartilage bones, however, is more complicated. Figure XVII., represents, in a diagrammatic way, the stages in the conversion of a cartilaginous bar to bone. To begin with, the previously sporadically-arranged (scattered anyhow) corpuscles (u.c.c.) are gathered into groups in single file, or in other words, into "columnar" groups (as at c.c.). The matrix becomes clouded with inorganic salts of lime, and it is then said to be calcified. This calcified cartilage then undergoes absorption-- it must not be imagined for a moment that bone is calcified cartilage. Simultaneous with the formation of the cavities (s.) due to this absorption, connective tissue (p.c.i.) from the surrounding perichondrium (p.c.) grows into the ossifying* bar. It is from this connective tissue that the osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is continually being absorbed and reformed by the activity of the osteoblasts. An osteoblast engaged in the absorption instead of the formation of bone is called an osteoclast. * The formation of bone is called ossification. To ossify is to become bony. Section 72. The great thing to notice about this is that cartilage does not become bone, but is eaten into and ousted by it; the osteoblasts and osteoclasts replace entirely the cartilage corpuscles, and are not derived from them.