20 



OUR BODIES AND HOW WE LIVE 



FIG. 13. Cross-Section from 

 Shaft of a Long Bone. 



Magnified 56 times. 



Little openings (Haversian canals) 

 are seen, and around them are 

 arranged rings of bone with lit- 

 tle dark spaces (lacuncs), from 

 which branch out fine dark lines 

 (canalictilf). 



are of various odd shapes and sizes, 

 and hence are called irregular, as 

 the bones of the wrist, the skull, 

 and the ankle. 



29. How Bone looks under the Mi- 

 croscope. Bones have a great number 

 of very small blood vessels, which 

 pass through tiny canals, Haversian 

 canals, in the bones, carrying mate- 

 rials for their nourishment and 

 growth. Round these canals the 

 spiderlike bone cells lie in small cav- 

 ities, lacunae, which are arranged in 

 circles or rows. These cavities are 

 joined to one another and to the 

 canal which they surround, by other 

 extremely fine canals, canaliculi. The 

 nourishing fluid of the blood, lymph, 

 soaks through these minute cavities 

 to all parts of the bone. 



The hard part of the bone lies 

 around the Haversian canals in lay- 

 ers, lamellae, which are interrupted at 

 intervals by the lacunas and canaliculi. 



When a thin section of hard, dry 

 bone is examined under the micro- 

 scope, the Haversian canals are seen 

 as holes surrounded by lamellae of 

 bone. The lacunae and canaliculi, 

 filled with dust and air, appear as 

 dots and lines. The bone cells and 

 marrow have entirely disappeared. 



NOTE. A very delicate layer of 

 connective tissue, called the periosteum, 

 closely adheres to every part of the 



