64 NORMAL HISTOLOGY. 



periosteum; and, 3, the marrow contained in the 

 central cavities or spaces within. 



i. The most striking feature of bone-tissue proper 

 is its firmness and hardness, which is due to the 

 presence of certain inorganic salts of lime. Various 

 acids dissolve these lime salts, and when they are 

 removed a substance is left behind which retains, 

 for the most part, both in general form and minute 

 structure, all the essential features of the original 

 bone. There is a basement substance, presenting 

 many of the optical characters of hyaline cartilage ; 

 and lying in tiny, branching spaces, in the basement 

 substance, are flat, nucleated cells. The lime salts 

 are deposited in the intercellular substance in such 

 an extremely minute state of division as to be in- 

 visible, even with high powers of the microscope. 

 The elongated and flattened cell-spaces of bone are 

 frequently called lacunce, and the numerous fine, 

 branching, intercommunicating channels .which pass 

 off from them in all directions, and open into the 

 narrow cavities, or into the passages for the blood- 

 vessels, are called canaliculi. 



The bone-cells, or bone-corpuscles, as they were 

 formerly called, are, in adults, thin, flat cells, with 

 spheroidal or oval projecting nuclei. It was formerly 

 believed that they sent, fine branching processes off 

 into the ramifications of the canaliculi. Recent 

 investigations, however, have thrown great doubt 

 upon the existence of at least such numerous pro- 



