242 MANUAL OF HISTOLOGY. 



pendicularly, only that the horizontal passages of communication are 

 enveloped by corresponding lamellae. The latter may be best seen, 

 though seldom, in the horizontal canals occurring in transverse sections, 

 cut through in their length. 



In other parts of the skeleton this beautiful regularity is less marked. 

 Thus we see, even in the epiphyses of the cylindrical bones, that these 

 systems of lamellae are much less developed, that the medullary canals 

 are enclosed within an inconsiderable number of the latter, while the 

 more internal general lamellae are entirely missing. In spongy osseous 

 tissue the laminated texture is rather more apparent in thick bands and 

 plates, while it disappears more and more as the latter diminish in volume. 

 In the outer layers of flat bones the general lamellae, as well as the Haver- 

 sian canals with theirs, run parallel to the surface. The same may be 

 remarked with both systems in the compact layer covering the short bones. 

 The great energy of the formative process in young bone often effects a 

 re-solution of already perfect tissue, commencing in one of the Haversian 

 canal-systems (fig. 234, a). This produces irregularly-bounded cavities of 

 varying size, with eroded edges, and lamellae appearing as though gnawed 

 away at points. Tomes and De Morgan, who first directed attention to 

 this, have given to these the name of "Haversian spaces-." 



S : 7 Trans j e r s section of a human phalanx. *, 7/aversian system of the ordinary kind 

 a a two others which have undergone re-absorption in the interior (b b) thus giving rise to Ha 

 Persian spaces, which are filled anew with lamella; c , another such system in *vhich re absoS 





Such a cavity may be subsequently filled up by a new system of special 

 lamellae, its characteristic outline nevertheless betraying its origin (b b} 

 Indeed, as I myself saw, some years ago, in a human phalanx, one of these 

 systems ^occupymg an Haversian space may undergo re-absorption for the 

 second time from the centre, with a tertiary formation of concentric lamellae 

 n its interior (c). Haversian spaces of this kind are of no very rare occur- 

 rence When present in large number, they may impart to the bone con- 

 siderable irregularity of texture. 



142. 



Osseous substance, which may be numbered among the double refract- 

 in- tissues, as the polarisation microscope teaches, has a rather homoge- 



