MICROSCOPIC STRUCTURE OF BONE 



43 



to supply blood to the solid part of the bone. The long bones are supplied 

 also by a proper nutrient artery which, entering at some part of the shaft 

 so as to reach the medullary canal, breaks up into branches for the supply 

 of the marrow, from which again small vessels are distributed to the interior 

 of the bone. Other small nutrient vessels pierce the articular extremities 

 for the supply of the cancellous tissue. 



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 



Fio. 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 visi- 

 ble; 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 



