210 THE ANIMAL KINGDOM. 



CHAPTER XI. 



SECTIONS OF BONE. 



THESE are favorite objects for the polariscope, and are 

 usually selected from their brilliancy under polarized light ; 

 but the structure of bone is a most interesting study as 

 connected with comparative anatomy and geological re- 

 searches, opening a wide field of observation. 



Bone is formed, like all other parts of the body, by the 

 development of cells, in which secondary deposits of earthy 

 or inorganic matter consolidate the tissue and form the 

 substance. Chemically, bone consists of gelatine, with 

 phosphate of lime, carbonate of magnesia, fluoride of cal- 

 cium, small quantities of carbonate of lime, and a little 

 oxide of iron. 



The marrow or medullary tissue of bones consists of 

 ordinary fatty tissue, a particular liquid, and cells, with 

 vessels and nerves. 



The structure will only be understood by the examination 

 of a few of these slides. Take, for example, a section of 

 human bone, 



The first thing we notice is the number of apertures 

 surrounded by laminae or layers of substance in circles. 

 These are the Haversian canals which serve for the trans- 

 mission of blood-vessels to the interior of the bone. The 

 numerous black spots with radiating fibres are called lacuna, 

 or bone-cells, and the fine lines are little tubes called 

 canaliculi, or calcigerous canals. They are dark, because 

 filled with air, and their shape and size are most important 

 matters to the naturalist, who thereby can determine to 

 what class of Bird, Beast, Eeptile, or Pish, any given bone 

 belongs. 



Not only so, but by the arrangement of the Haversian 

 canals and bone-cells, differing in every bone of the body, 

 from the bones of man to those of the smallest creature, 



