694 



MINUTE STRUCTURE OF BONE. 



perfect type presented to us in the lower Vertebrata, the whole 

 thickness is usually more or less ' cancellated,' that is, burrowed 

 out by medullary cavities. When we examine, under a low mag- 

 nifying power, a longitudinal section of a long Bone, or a section 

 of a flat Bone parallel to its surface, we find it traversed by 

 numerous canals, termed Haversian 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 irre- 

 gular network, — On applying a higher magnifying power to a thin 

 transverse section of a long Bone, we observe that each of the 

 canals whose oi'ifices present themselves in the field of view (Fig. 

 355). is the centre of a rod of bony tissue (1), usually more or less 



Fig. 355. 



Minute structure of Bone, as seen in transverse section :— 1, a 

 rod surrounding an Haversian canal, 3, showing the concentric 

 arrangement of the lamellae; 2, the same, with the lacunae and 

 canaliculi ; 4, portions of the lamellse parallel with the external 

 surface. 



circular in its form, which is arranged around it in concentric rings, 

 resembling those of an Exogenous Stem. These rings are marked 

 out and divided 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 extremely 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 direction of its circum- 

 ference ; and by the inosculation of the tubules (which are termed 



