THE OSSEOUS SYSTEM. 



277 



communicate with the vascular canals, the medullary cavities, and the 

 medullary spaces or cancelli of the spongy substance, or open on the 

 surface of the bone. The entire osseous substance, therefore, is pene- 

 trated ly a connected system of cavities and canaliculi, by means of 

 which the nutritive juice secreted by the vessels is conveyed into its 

 densest tissue. 



The lacunae and canaliculi do not exhibit precisely the same condi- 

 tions in every part of the bones. In the lamellar systems of the Ha- 

 versian canals, as seen in a transverse section, the elongated lacunae, 

 by reason of their curvature, lie as it were concentric to the canal, and 

 their excessively numerous pores or canaliculi necessarily produce a 

 very close striation radiating from the vascular canal (Fig. 115). 

 The lacunae are sometimes extremely numerous, sometimes more 

 scanty; in the former case they are, for the most part, arranged 

 in tolerably regular alternation, or one behind the other in the 

 direction of the radius of the lamellar system ; but they are also 

 frequently disposed very irregularly, either crowded together (vide 

 the lower part of Fig. 115), or separated by wider interspaces. In 

 horizontal and longitudinal sections of Haversian canals (Fig. 116), 

 when the section has passed through the middle of a canal, the 

 lacunae appear narrow and elongated, and disposed in rows one be- 

 hind the other, and in numerous layers parallel with the canal ; and 

 also furnished with numerous canaliculi, which proceed for the most part 



Fig. 117. 



directly inwards and outwards (consequently transversely through the 

 lamellae), but partly in a direction parallel with the long axis of the 



FIG. 117. Lacunae viewed on the flat side, with the canaliculi, from the parietal bone; 

 magnified 450 diam.: the spots on the lacuna? or between them belong to canaliculi, which 

 are cut across, or are the openings of canaliculi into the lacunae ; a a a, groups of transverse 

 sections of canaliculi, each group belonging to a lacuna which has been destroyed in the 

 making of the section. 



