xcii BONE. 



freely open into one another, their oblique communications connecting them 

 both longitudinally and laterally. Those also which are next the circum- 

 ference of the bone, open by minute pores on its external surface, and the 

 innermost ones open widely into the medullary cavity ; so that these short 

 channels collectively form a sort of irregular network of tubes running 

 through the compact tissue, in which the vessels of that tissue are lodged, 

 and through the medium of which these vessels communicate together, not 

 only along the length of the bone, but from its surface to the interior, 

 through the thickness of the shaft. The canals of the compact tissue in 

 the other classes of bones have the same general characters, and for the 

 most part run parallel to the surface. 



On viewing a thin transverse section of a long bone with a microscope 

 of moderate power, especially after the earthy part has been removed by 

 acid (fig. XLIII. B), the opening of each Haversian canal appears to be sur- 

 rounded by a series of concentric rings. This appearance is occasioned by 

 the transverse sections of concentric lamella which surround the canals. 

 The rings are not all complete, for here and there one may be seen ending 

 between two others. In some of the sets the rings are nearly circular, in 

 others oval, differences which seem mostly to depend on the direction in 

 which the canal happens to be cut : the aperture, too, may be in the centre, 

 or more or less to one side, and in the latter case the rings are usually 

 narrower and closer together on the side towards which the aperture deviates. 

 Again, some of the apertures are much lengthened or angular in shape, and 

 the lamellae surrounding them have a corresponding disposition. Besides 

 the lamellae surrounding the Haversian canals, there are others disposed 

 conformably with the circumference of the bone (fig. XLIII. B, a), and which 

 may therefore be said to be concentric with the medullary canal ; some of 

 these are near the surface of the bone, others run between the Haversian 

 sets, by which they are interrupted in many places. Lastly, in various 

 parts of the section, lines are seen which indicate lamellae, differing in 

 direction from both of the above-mentioned orders. As to the circum- 

 ferential laminae, Messrs. Tomes and De Morgan state that they are by no 

 means so common as is generally supposed ; further, that they are most 

 conspicuous in bones of full growth, in which, consequently, nutritive 

 changes proceed slowly ; and that their presence may be made the means of 

 determining, within certain limits, the age at which a bone has arrived. 

 These authors observe, that in young and rapidly growing bones the lamina) 

 are frequently seen to have an undulating direction, which they consider as 

 a sign that the tissue is undergoing rapid nutritive changes. 



The appearance in a longitudinal section of the bone is in harmony with 

 the account above given : the sections of the lamellae are seen as straight 

 and parallel lines, running in the longitudinal direction of the bone, except 

 when the section happens to have passed directly or slantingly across a 

 canal ; for wherever this occurs there is seen, as in a transverse section, a 

 series of rings, generally oval and much lengthened on account of the obli- 

 quity of the section. 



The cancellated texture has essentially the same lamellar structure. The 

 slender bony walls of its little cavities or areolse are made up of super- 

 imposed lamellae, like those of the Haversian canals (fig. XLIII. B, b) t only 

 they have fewer lamellae in proportion to the width of the cavities which 

 they surround ; and, indeed, the relative amount of solid matter and open 

 space constitutes, as already said, the only difference between the two forms 

 of bony tissue ; the intimate structure of the solid substance and the 



