NORMAL HISTOLOGY. 



vessels. Between the annular lamellae are seen small spindle-shaped or oval 

 spaces, the lacuna (about 20 t* long, 10 A wide and 6 n thick), from which 

 minute passages, the canaliculi, radiate and join with others to establish 

 communication between the adjacent lacunae of the same Haversian system. 

 The lacunae and canaliculi thus form an intercommunicating network of 

 lymph-spaces similar to those in other dense connective tissue. When 

 viewed in profile, as they are in sections cutting the lamellae at right angles, 

 the lacunae present their smaller dimensions and appear as minute fusiform 





'&: External 

 s *5?v circumferential lamellae 



^ ^^^^a^is^w 

 SKH^^^S^^^ 



Internal 

 " circumferential lamella 



FIG. 49. Transverse section of compact bone; the section has been ground and dried, hence the lacunae 



are filled with air. X 70. 



spaces; seen in sections which pass parallel to the lamellae (Fig. 50), the 

 lacunae are broader and more circular, the spaces with the canaliculi forming 

 the spider-like figures so conspicuous in sections of dried bone. 



The characteristic disposition of the lamellae of the Haversian system is 

 due to the secondary formation of the bone-tissue during the conversion of 

 the spongy bone into the compact, the circumference of each system cor- 

 responding to an Haversian space, the cavity in which the connective 

 lamellae were deposited. It follows, from this relation, that the Haversian 

 systems exist only in compact bone, the secondary deposit not occurring 

 during the development of cancellated bone. When deprived of the min- 



