THE COMPOSITION AND STRUCTURE OF BONE 43 



surface is covered with a tough and fibrous membrane, called 

 periosteum (Gr. peri, around, and osteon, a bone). 



Blood is supplied to the bone by small vessels from this 

 periosteum, which pierce the bone underneath it. Other small 

 vessels pierce the articular extremities, and pass to the cancellous 

 tissue. 



The circulation of blood within the substance of the bone is 

 brought about by means of a number of minute canals, which are 

 to be found in all parts of the bone. 



Fig. 48. Section parallel to the surface from the Shaft of the Femur. 

 Magnified 100 diameters. 



a, Haversian canals ; b, c, lacunae. 



In order to study the minute structure of bone it will be necessary to cut 

 a very thin slice from the compact tissue, and grind this down till it is semi- 

 transparent. If the cutting be made transversely, it will show, under the 

 microscope, cross sections of minute canals, called Haversian canals from 

 Havers, who first discovered them. These canals vary in size, the average 

 diameter being about ^ of an inch. The Haversian canals are surrounded 

 by small, irregular spaces called lacunae (Lat. cavities), arranged in con- 

 centric circles, and so giving the bone a laminated appearance. The lacuna? 



