HISTOLOGY. 



"'"T.W 



mesenchyma. The osteoblasts may be enclosed by the matrix which they 

 form, thus becoming bone cells. The trabeculae of bone produced in 

 this manner unite in a network, described as spongy bone. By the deposi- 

 tion of new layers or lamellae of bone, which conform with the surfaces 

 on which they are laid down, the spongy bone becomes compact. By 

 resorption of the inner part, the marrow cavity forms and parts of the 

 compact bone become spongy. It remains to consider the substances 

 and appearances of adult bone, and to describe the manner in which the 

 cartilages are replaced by bone. 



The matrix of bone is at first uncalcified and soft, apparently homo- 

 geneous, but actually con- 

 sisting of cemented fibrils. 

 It consists chiefly of col- 

 lagen the gelatin-pro- 

 ducing substance, and of 

 a mucoid called osseo- 

 mucoid. Through it there 

 may be distributed fine 

 elastic fibers (said to be 

 lacking in the bones of 

 the vertex of the skull) 

 besides the coarser con- 

 nective tissue bundles of 

 Sharpey. Soon after this 

 organic matrix is estab- 

 lished, calcification be- 

 gins by the deposition of 

 lime salts either in or be- 

 tween the fibrils. Over 

 80% of the inorganic mat- 

 ter is calcium phosphate, 

 Ca 3 (PO 4 ) 2 , the remainder including chlorides, carbonates, fluorides and 

 sulphates of calcium, sodium, potassium, and magnesium. The proper- 

 ties of bone depend largely upon the intimate blending of the organic 

 and inorganic constituents, possibly in chemical combination. The two 

 parts may be separated, however. Acids remove the salts leaving the 

 organic portion as a flexible counterpart of the entire bone. Heat or 

 maceration may be employed to destroy the organic part. Microscopic 

 preparations are made in either way, but usually from decalcified bones. 

 All of the drawings thus far referred to were of such specimens. 



The cross section of a decalcified long bone of an adult, Fig. 621 



Fat drops. 



FIG. 63. FROM A LONGITUDINAL SECTION OF A HUMAN META- 



CARPAL. X 30. 

 Fat drops are seen in the Haversian canals. At x Haversian 



canals open on the outer, and at zz on thf inner surface of 



the bone. 



