STRUCTUEE. S3 



century, who more especially called attention to them. Blood-vessels 

 run in these canals, and the widest of them also contain marrow. They 

 are from xoVotli to -^00^^^ of ^n inch in diameter: there are some 

 no more than -^Q^jy-oth, but these are rare ; the medium size is about 

 -=-^th. The widest are those nearest the medullary cavity, and they 

 much smaller towards the circumference of the bone. They are 

 quite short, as may be seen in a longitudinal section, and somewhat 

 crooked or oblique at their ends, where they freely open into one 

 another, their oblique communications connecting them both longi- 

 tudinally and laterally. Those also 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 commu- 

 nicate 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 micro- 

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

 removed by acid (fig. 48 b), the opening of each Haversian canal 

 appears to be surrounded by a series of concentric rings. This ap- 

 pearance is occasioned by the transverse sections of concentric lamellae 

 which surround the canals. The rings are not all complete, for here 

 <and tliere 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 

 lamellre 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. 48 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 botli of the above-mentioned orders. As to 

 the circumferential laminae, Tomes and De IMorgan state that they are 

 by no means so common as is generally supposed ; further, i:hat 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. 



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

 with the account above given : the sections of the lamellce are seen as 

 straight and parallel lines, running in the longitudinal direction of 

 the bone, except when the section happens to have ]3assed 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. 



G 2 



