40 



ELEMENTS OF PHYSIOLOGY 



nourishment. It is closely adherent to the bone and may be 

 removed by scraping. It is sometimes seen as a ragged 

 margin on the sawed end of a bone fresh from the butcher. 

 65. The hardest and most compact part of a bone seen 

 under the microscope shows the following system of open- 

 ings (Fig. 44): (i) canals, called Haversian canals, run- 

 ning lengthwise of the bone and containing blood vessels 



which extend into them 

 from the periosteum ; 

 (2) irregular spaces called 

 lacuna arranged in cir- 

 cular lines around the 

 canals; (3") from the la- 

 cunae numerous minute 

 wavy passages called 

 canaliculi pass inward 

 to the Haversian canals, 

 while others pass out- 

 ward to open into the 

 lacunas in the outer cir- 

 cles. The canaliculi 



afford P assa S e to minute 

 blood vessels. 



66. The lacunae con- 

 tain small cells called 

 bone cells, each having a 

 nucleus and sending out 

 fine processes or branches 



for some distance along the canaliculi. They receive their 

 nourishment from the blood flowing through the Haversian 

 canals, which penetrate to them in their stony prisons, 

 through the canaliculi. They have for their special form 

 of activity the building of walls of phosphate and carbonate 

 of lime about themselves. The walls built by the neigh- 

 boring cells touch one another, and becoming fused together 



FIG. 44. Bone cut across, highly 

 magnified. 



H, Haversian canals ; /, lacunae, connected by 

 canaliculi. 



