28 



COMPARATIYE ANATOMY. 



Fig. 11. 



Eana. o 

 bone-cells 



found in the skeletal organs of all classes of tlie Vertebrata ; whilst 

 that form of osseous tissue witli canaliculi only is found in the 

 skeleton of many fishes, and as a general rule in the dental organs 

 of all Vertebrata (dentine). 



The development of osseous tissue explains the relations of the 



intercellular substance to 

 the cells. That form of it 

 which contains cells may 

 arise in two ways : either 

 by the ossification of con- 

 nective tissue, the cells in 

 which become converted 

 into bone-cells by the ossi- 

 fication of the intercellular 

 substance, which becomes 

 impregnated with calca- 

 reous salts, while the cells 

 themselves become con- 

 nected with one another 

 by their processes, which 

 traverse the pore-canals ia 

 the intercellular substance; 

 or the same tissue is formed 

 by apparently indifferent 

 cells, which secrete a scle- 

 rogenous substance. This 

 substance is laid down in stratified lamellae, into which the secreting 

 cells send fine protoplasmic processes (Fig. 11, o). The secretion 

 of this substance is preceded by a change of part of the protoplasm 

 of the cell. As soon as this is differentiated it does not belong 

 any longer to the cell, and is therefore secreted from it. If some 

 of the secreting cells (o'o") cease to be active, while the cells near 

 them do not cease to be so, the former gradually get to lie in a 

 layer of intercellular substance, which finally surrounds them, and 

 so converts them into bone-cells (o'"). The cells of the secreting 

 layer (osteoblasts) are continuously connected by fine processes with 

 those which are already enclosed (bone-cells). Thus each of the 

 former is rendered capable of becoming a bone-cell. 



The other form of osseous tissue is developed in a perfectly 

 analogous manner, so far as its history is accurately known, through 

 the development of dentine. In this case also a layer of cells 

 secretes a substance, which hardens or is sclerogenous, and into this 

 the cells at the same time send processes, which traverse pore- 

 canals. But the cells (odontoblasts), instead of gradually sinking 

 into this extra-cellular substance, always remain outside it, and are 

 connected with it by their processes only. The secreting substance 

 is thus ti^versed by fine parallel canalicuH (the dentinal canals, 

 so-called, since they were first made out in the dentine). This form of 

 osseous tissue, notwithstanding its different appearance in later stages, 



Transverse section of the femnr of 

 Osteoblast layer. o'o"Cells becoming 



o'" A bone-cell. p Periosteum. 



m Medullary cavity. 



