2 PRACTICAL ANATOMY. 



while the hyoid bone and lower ribs contain a larger 

 percentage of organic matter than the average. The 

 .specific gravity of bone varies, the average being near 

 1.85. 



On section, bone is seen to be composed of two kinds 

 of tissue, an outer dense portion, or compact bone, and 

 an inner portion, which is loose, spongy, and cancellous. 

 An examination of the compact tissue by means of a 

 suitable lens demonstrates that it is exceedingly porous; 

 in fact, the difference between the compact and cancel- 

 lous tissue is largely one of condensation. These two 

 tissues are found in different amounts in different bones, 

 according to the requirements of the structures into 

 which they enter. Thus, where great bulk and little 

 weight are important, the spongy tissue predominates, 

 surrounded by a shell of compact tissue, which is always 

 on the exterior. If a living bone is cut, it bleeds, owing 

 to the free vascular supply derived from the periosteum ; 

 this is especially the case in the growing skeleton, where 

 the osseous circulation is very active. The blood-vessels 

 which enter the bone penetrate at all points on its sur- 

 face, and then run in minute canals in the substance of 

 the bone. If the periosteum is destroyed, either by 

 disease or injury, caries always, and necrosis sometimes, 

 results. 



MINUTE ANATOMY OF BONE. 



If a thin, transverse section of a long bone is exam- 

 ined through a ^ or ^ objective, its structure is clearly 

 brought into view. Large, round openings are observed, 

 which are the cut ends of the Haversian canals, and 

 around these are clustered concentrically minute, dark, 

 oval cavities, called lacunae. A number of fine canals 

 are seen, passing from lacuna to lacuna, and from these 

 to the Haversian canals. They are known as the canalic- 



