MINUTE STRUCTURE OP BONE. 257 



rare ; the medium size is about -^th. The widest are met with nearest the 

 medullary cavity, and the narrower towards the circumference of the bone. They 

 are quite short, as may be seen in a longitudinal section, oblique communications 

 connecting them freely both longitudinally and laterally. Those which are next the 

 circumference 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. 



Fig. 297. SECTION OF A HAVERSIAN CANAL, SHOWING ITS 

 CONTENTS. HIGHLY MAGNIFIED. (E. A. S.) 



a, small arterial capillary vessel ; v, large venous capil- 

 lary ; n, pale nerve-fibres cut across : I, cleft-like lymph- 

 atic vessel : one of the cells forming its wall communicates 

 by fine branches with the branches of a bone-corpuscle. 



The substance in which the vessels run is connective tissue f / JT"~^MOt^l B S 7 



with ramified cells ; its finely granular appearance is pro- | | \ jgj HJBflriTm' m~ 



bably due to the cross section of fine fibrils. 



Most of the Haversian canals contain two 

 small blood-vessels, arterial and venous (fig. 297), 

 together with a small amount of delicate con- 

 nective tissue containing branched cells, which 

 are flattened close to the bone, and com- 

 municate by their branches with the ramifi- 

 cations of corpuscles in the substance of the bone. 



Lamellae. On viewing a thin transverse section of a long bone with a micro- 

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

 (fig. 296 B ; fig. 297), the opening of each Haversian canal appears to be surrounded 

 by a series of concentric rings. This appearance is occasioned by the transverse 

 sections of concentric lamellce 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, Home of the apertures are much lengthened or angular in shape, 

 and the lamellae surrounding them have a corresponding disposition. Besides the 

 lamellse surrounding the Haversian canals, there are others disposed conformably 

 with the circumference of the bone (fig. 296 B, a) ; most of these are near the 

 surface, but others run between the Haversian sets, by which they are interrupted in 

 many places (fig. 303). Lastly, in various parts of the section, lines are seen which 

 indicate lamellae, differing in direction from both of the above-mentioned orders. 



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

 account above given : the sections of the lamellse 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 obliquity of the section. 



Many of the Haversian canals which pass through the circumferential or periosteal lamellae 

 carrying blood-vessels from the periosteum into the bone, are not surrounded by concentrie 



