BONE. 299 



appear like so many small dots ; they contain also, scattered 

 throughout their substance, multitudes of granules of earthy 

 matter. 



The only really necessary constituents of bone would appear 

 to be the cellular tissue and earthy matter. The combination 

 of these two forms bone in its simplest condition. The me- 

 dullary cells, Haversian canals, and bone cells, are connected 

 only with the growth and nutrition of bone and occur seldom, 

 except where the size of the bony formation renders their 

 presence necessary for its growth and support. 



Bone Cells. 



Distributed throughout the cancellous and the compact 

 portions of bone, cells of a peculiar structure occur in con- 

 siderable quantities. 



Concerning the nature of these cells much difference of 

 opinion prevails ; by some they are described as mere vacuities 

 existing in the tissue of the bone, by others as hollow cells, 

 as nuclei of cells, and as true nucleated corpuscles. That 

 the bone cells take their origin in nucleated cells cannot 

 be doubted. 



The circumstances which have given rise to the notion of 

 their being mere vacancies or lacunae are the passage of 

 fluids through them, their infiltration with solid matter, and 

 the optical appearances sometimes presented by them. All 

 these circumstances admit, however, of explanation on the 

 supposition of their corpuscular origin. 



That they are derived from granular cells may be proved, 

 it seems to me, by the study of the development of bone, 

 they being in growing bones first traceable as nucleated cor- 

 puscles, a condition to which they may be again reduced in 

 an adult bone by the removal of the earthy matter. This 

 appearance is best seen in the large bone cells of the Siren, 

 Proteus, or Menobranchus. 



The bone cells, which are very numerous, are situated be- 



B B 3 



