310 ELEMENTARY PHYSIOLOGY less. 



intervals round each Haversian canal (Fig. 94). The hard 

 bony tissue appears therefore to be composed of lamellae, 

 which are disposed concentrically around the Haversian 

 canals ; and a Haversian canal with the concentric 

 lamellae belonging to it form what is called a Haversian 

 system. The soft substance from which the bone-earth 

 has been extracted is similarly lamellated, and here and 

 there presents fibres which may be traced into the fibrous 

 substance of the periosteum. 



If a thin section of dry bone is examined with the 

 microscope (Fig. 95), by transmitted light, each dark spot 

 is seen to be a black body (of an average diameter of 

 about lofj.) with an irregular jagged outline, and pro- 

 ceeding from it are numerous fine dark lines which 

 ramify in the surrounding matrix and unite with similar 

 branched lines from adjacent black bodies. The matrix 

 itself has a somewhat granular aspect. In a transverse 

 section these black bodies are rounded or oval in form, 

 but in a longitudinal section they appear almost spindle- 

 shaped ; that is to say they are lenticular or lens-shaped ; 

 but flattened as it were between the adjacent layers of 

 the matrix. Examined by reflected light the same bodies 

 look white and glistening ; and if the section instead of 

 being examined dry, be boiled in water or soaked in strong 

 alcohol, and brought under the microscope while still wet, 

 the black bodies with their branching lines will be found 

 to have almost disappeared, only faint outlines of them 

 being left. At the same time minute bubbles of air will 

 have escaped from the section. The black bodies seen in 

 the dry bone are in fact "lacunae," i.e. gaps, or holes in 

 the solid matrix, appearing black by transmitted light 

 and white by reflected light, because they are filled 

 with air ; and the dark branched lines are similarly, 

 minute canals, " canaliculi," also filled with air- 

 bubbles, drawn out so to speak into lines, also 

 hollowed out of the solid matrix, and placing one lacuna 

 in communication with another. In each Haversian 



