STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 51 



by containing a great quantity of earthy or inorganic matter, 

 which gives the tissue its enormous strength. It is, moreover, 

 everywhere traversed by the processes of the cells lying in little 



FIG. 33. 



Transverse section of a system of Havers, showing Haversian canal in 

 centre, with bone cells arranged around it in lacunae, which are connected 

 by the delicate canaliculi, (Cadiat.) 



canals (canaliculi), which connect the spaces (lacunse) in which 

 the protoplasmic bone cells sojourn. 



In the formation of bone from fibrous or cartilaginous tissue 

 the original intercellular substance disappears, and a set of cells 

 with new formative powers come upon the field (Fig. 34). These 

 new cells (osteoblasts) cover the growing surface of the bone and 

 secrete and lay down in layers a new kind of intercellular sub- 

 stance, which is the bone matrix. Here and there at wonderfully 

 regular intervals an osteoblast ceases to secrete the calcareous inter- 

 cellular substance, while its neighbors continue formative activity. 

 Consequently, this osteoblast, or as it may now be called young 

 bone cell, becomes surrounded by calcareous intercellular sub- 

 stance, and is thus permanently lodged in the bone tissue. 



