BONE 61 



Very little basophilic matrix is formed and it ai)pears intermediate 

 between the dense fibrous connective tissue of perichondrium and 

 hyaline cartilat2;e; the intercellular material resembles that in 

 perichondrium or tendons, but the cells are ovoid and characteristic 

 of cartilage. It occurs, for example, covering the surfaces of 

 vertebrae where they make contact. 



BONE. 



This is the last type of connecti\e tissue to make its appearance 

 during embryonic development and is also the hardest tissue. The 

 skeleton of the majority of the higher vertebrates is first laid down 

 as a cartilage but is later replaced by bone, although some bones 

 arise directly from dift'erentiation of mesenchyme cells of the 

 embryo. In each case the nature of the bone formed is the same 

 but the processes involved are separated into a direct or intra- 

 membranous ossification, and an indirect or endochondral ossifica- 

 tion in which the bone is preceded by a cartilage mass. The special- 

 ized connective-tissue cells through whose actiN'ity the matrix of 

 bone is formed are called osteoblasts. As a result of intercellular 

 deposits these cells become embedded in matrix and occupy spaces, 

 or lacuna^, in the matrix. Processes of their cytoplasm extend in 

 small channels, or canaliculi, to make contact with adjacent cells. 



Bones are composed of about 'M) per cent organic and 70 j^er cent 

 inorganic material. The inorganic material consists chief!}' of 

 calcium phosphate, with small amounts of calcium carbonate, mag- 

 nesium phosphate, and sodium chlorifle. \Yhen fresh bones are 

 boiled in water, a gelatinous organic substance called ostein is 

 obtained, and is similar to the collagen deri\-ed from other ty])es of 

 connective tissues. If fresh bones are placed for a long period in a 

 weak solution of nitric or hydrochloric acid, the inorganic material 

 is removed and a flexible decalcified bone is left. Heating bone to 

 red heat destroys the organic matter and leaves a hard, brittle 

 substance. Structurally, bone consists of small bundles of collagen- 

 ous fibers impregnated with calcium salts. 



Intramembranous Ossification. — The flat bones of the cranium and 

 face develop directly from sheets of mesenchyme tissue in which 

 ossification begins at one or more central ])olnts. At these ])()ints, 

 mesenchyme cells ditt'erentiate into osteoblasts which dei)osit 

 acidophilic, fibrillar, intercellular substance. Calcium salts are 

 deposited in this matrix w^hich increases in amount to form spicules 



