44 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



Microscopic Structure of Bone. Notwithstanding the differences 

 of arrangement just mentioned, the structure of all compact bone substance 

 is found under the microscope to be essentially the same. 



Examined with a rather high power its substance is found to contain a 

 multitude of small irregular spaces, approximately fusiform in shape, called 

 lacuna, with very minute canals or canaliculi, as they are termed, leading 



FIG. 54. Longitudinal Section from the Human Ulna, Showing Haversian Canals, 

 Lacunae, and Canaliculi. (Rollett.) 



from them, and anastomosing with similar prolongations from other lacunae, 

 figure 53. In very thin layers of bone, no other canals than these may be vis- 

 ible; but on making a transverse section of the compact tissue of a long bone, 

 as the humerus or ulna, the arrangement shown in figure 53 can be seen. 

 The bone seems mapped out into small circular districts, at or about the 

 center of each of which is a hole, around which are concentric layers, the 

 lamella, the lacuna and canaliculi following the same concentric distribution 

 around the center, with which indeed they communicate. 



On making a longitudinal section, the central holes are shown to be 

 simply the cut extremities of small canals which run lengthwise through 

 the bone, anastomosing with each other by lateral branches, figure 54, and 

 are called Haversian Canals, after the name of the physician, Clopton Havers, 

 who first accurately described them. 



The Haversian Canals. The average diameter of the Haversian canals 

 is 5o/. They contain blood vessels, and by means of them blood is con- 

 veyed to even the densest parts of the bone; the minute canaliculi and lacunae 



