BONE. 



them, vary from y-^j- to -% J^ of an inch in diameter ; they 

 enter from the surface of the bone by multitudes of minute 

 oblique openings visible with the naked eye, and run for the 

 most part longitudinally, connected however by numerous 

 short canals, which have a more transverse direction. The 



Fig. 22. LACUNAE AND Fig. 23. TRANSVERSE SECTION OB= 

 CANALICULI. COMPACT OSSEOUS TISSUE, a, 



Haversian canal ; &, Lacunce in 

 concentric rings. 



tissue is arranged in concentric laminae around the Haversian 

 canals, so that circles of lacunae are seen surrounding the 

 transverse sections of the canals, and such an arrangement of 

 concentric rings is called an Haversian system. The whole 

 compact tissue is made up of such systems, the interstices 

 being filled with fragments of similar laminae which were 

 formerly complete, but of which the other portions have 

 been absorbed so as to leave gaps or absorption-spaces, sub- 

 sequently filled up by new systems developed concentrically 

 from the circumference inwards, till they have closely grasped 

 the blood-vessels in the centre. 



The arteries for the supply of bone subdivide in the 

 fibrous membrane by which each bone is surrounded, the 

 periosteum, and from this membrane small branches pass all 

 over the surface into the openings of the Haversian canals. 

 The veins emerge by comparatively few orifices of larger size, 



