34 MICROSCOPIC OBJECTS. 



their cells and fibres, a hard solid substance is produced. 

 In the shells of the Echinodermata the deposition of 

 carbonate of lime in fibres, while in Mollusca it is found 

 in cells. Sometimes the earthy matter crystallizes, as in 

 the teeth ; at other times it combines chemicallv with the 

 gelatine of the cells, as in bone. This deposition in bone 

 does not occur in all the cells, as the bone requires to 

 grow and be nourished : hence arises its peculiarity of 

 structure. Independently of the hoUows, or cancelli, the 

 hard part of the bone is traversed by canals called Ha- 

 versian, which run in the direction of the laminae : these 

 are connected together by transverse communications. 

 In a thin transverse section of bone the solid matter may 

 be observed arranged around the Haversian canals in con- 

 centric rows. Among these layers dark specks are dis- 

 persed. These dark specks (called lacuncej, when mag- 

 nified about 200 diameters, are observed to be cavities of 

 an irregular oval form, from which emanate numerous 

 minute branch canals. These cavities appear dark, for 

 the same reason as a minute air-bubble does in Canada 

 balsam — namely, the great difference of refraction of the 

 two media. By means of these branches fcanaliculijy 

 lacunae, and Haversian canals, the bone is nourished with 

 oily and other proper fluids. 



According to the microscopic observations of Mr. 

 Queckett, the form and size of these lacunse, and the 

 tortuosity of their canalicuh, vary in the bones of* differ- 

 ent kinds of animals ; so that a microscopic examination 

 of a minute fragment of bone is sufficient to enable the 



