STRUCTURE OF BONE 



IO2I 



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 irregular network. On apply- 

 ing a higher magnifying power to a thin transverse section of a long 

 bone we observe that each of the canals whose orifices present them- 

 selves in the field of view (fig. 751) is the centre of a rod of bony 

 tissue (1 ), usually more or less 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 out- 

 wards, or in the direction 

 of its circumference ; and 

 by the inosculation of the 

 tubules (or canaliculi) of 

 the different rings with 

 each other a continuous 

 communication is esta- 

 blished between the cen- 

 tral Haversian canal and 

 the outermost part of the 

 bony rod that surrounds 

 it, which doubtless minis- 

 ters to the nutrition 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 protoplasmic 

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

 in the lacunae into communication with the walls of the blood- 

 vessels. 



The minute cavities or lacunce from which the canaliculi proceed 

 (fig. 752) are highly characteristic of true osseous tissue, being never 

 deficient in the minutest parts of the bones of the higher Yertebrata, 

 although those of fishes are occasionally destitute of them. The dark 

 appearance which they present in sections of a dried bone is not due 

 to opacity, but is simply an optical effect, dependent (like the black- 

 ness of air-bubbles in liquids) upon the dispersion of the rays by the 

 highly refracting substance that surrounds them. The size and 

 form of the lacunae differ considerably in the several classes of Yer- 

 tebrata, and even in some instances in the orders, so that it is often 

 possible to determine the group to which a bone belonged by the 



FIG. 751. 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, portion of the 

 lamellse parallel with the external surface. 



