256 



THE MICROSCOPE AND ITS REVELATIONS. 



In the most developed kind of ' flat ' bones, again, such as those of the 

 head, we find the two surfaces to be composed of dense plates of bone, 

 with a 'cancellated' structure between them; whilst in the less perfect 

 type presented to us in the lower Vertebrata, the whole thickness is usu- 

 ally more or less ' cancellated/ that is, divided-up into minute medullary 

 cavities. When we examine, under a low magnifying power, a longitu- 

 dinal section of a long bone, or a section of a flat bone parallel to its 

 surface, we find it traversed by numerous canals, termed Haver sian after 

 their discoverer Havers, which are in connection with the central cavity, 

 and are filled, like it, with marrow: in the shafts of 'long' bones these 

 canals usually run in the direction of their length, but are connected 

 here and there by cross-branches; whilst in the flat-bones they form an 

 irregular network. On applying a higher magnifying power to a thin 

 transverse section of a long bone, we observe that each of the canals 

 whose orifices present themselves in the field of view (Fig. 440), is the 

 centre of a rod of bony tissue (1), usually more or less circular in its 



FIG. 440. 



FIG. 441. 



Minute structure of Bone, as seen in transverse 

 section:!, a rod surrounding an Haversian canal, 

 3, showing the concentric arrangement of the lam- 

 ellae; 2, the same, with the lacunae and canaliculi; 4, 

 portions of the lamellse parallel with the external 

 surface. 



ft 



Lacunce of Osseous substance: a, cen- 

 tral cavity; 6, its ramifications 



form, which is arranged around it in concentric rings, resembling those 

 of an Exogenus stem (Fig. 254). These rings are marked out and di- 

 vided by circles of little dark spots, which, when closely examined (2), 

 are seen to be minute flattened cavities excavated in the solid substance 

 of the bone, from the two flat sides of which pass forth a number of ex- 

 tremely minute tubules, one set extending inwards, or in the direction of 

 the centre of the system of rings, and the other outwards, or in the direc- 

 tion of its circumference; and by the inosculation of the tubules (or canali- 

 culi) of the different rings with each other, a continuous communication 

 is established between the central Haversian canal and the outermost part 

 of the bony rod that surrounds it, which doubtless ministers to the nu- 

 trition of the texture. Blood-vessels are traceable into the Haversian 

 canals, but the 'canaliculi' are far too minute to carry blood-corpuscles; 

 they are occupied, however, in the living bone, by threads of sarcodic 

 substance, which bring the segments of 'germinal matter' contained 

 in the lacunas into communication with the walls of the blood-vessels. 



