382 



THE MICROSCOPE. 



acid instead of caustic potash, when the earthy matter will be removed, 

 and the section will exhibit nearly the same form as when the earthy 

 constituent was present ; and when viewed microscopically, it will be 

 noticed that all the parts characterising the section previous to its ma- 

 ceration in the acid will be still visible, but not so distinct as when 

 both constituents were in combination. When, however, the animal 

 matter is removed, the bone will not exhibit the cells and the canali- 

 culi, but will be opaque and very brittle, and exhibit nothing but the 

 Haversian canals and a granular structure. 



If we consider what has been already mentioned as entering into 

 the composition of a bone, viz. the medullary cavity, the Haversian 

 canals, the canaliculi, and the bone -cells, we shall find that every part 

 thus described has been more or less hollow ; where, then, is the true 



1 2 



fig-. 182. 



1. A small portion of bone, taken from the exterior of the shaft of the htimerus of a 

 Pterodactyle, which exhibits the elongated bone-cells characteristic cf the orders of 

 Eeptilia. 



2. A horizontal section of a scale, or flattened spine, from the skin of a Trygon, or 

 Sting Ray, which exhibits large Haversian canals, with numerous wavy parallel 

 tubes, like those of dentine, communicating with them. It will be noticed that this 

 specimen shows, besides these wavy tubes, numerous bone-cells, whose canaliculi 

 communicate with the tubes, as in many specimens of dentine. 



bony substance 1 This is no other than the small granules of ossific 

 matter, which are situated between the canaliculi of the bone-cells, each 

 granule having an investment of soft animal matter, by which the whole 

 mass of granules is kept in firm apposition. 



The parts, then, which a transverse or a longitudinal section of a 

 long bone of a mammalian animal will exhibit, will be the Haversian 



