THE FROG 35 



exact models of the original bone. The decalcified one will be about 

 one-third the weight it was originally, and the burnt one about 

 two-thirds. The inorganic salts present in the matrix are mainly 

 Calcium phosphate, but small amounts of Calcium carbonate and 

 fluoride and Magnesium phosphate are also present. 



Most of the long bones are hollow, the central cavity in life being 

 filled with a soft red tissue, the marrow. This consists of a plentiful 

 supply of blood-vessels embedded in a fatty substance, in which 

 are special cells, the marrow cells, concerned with the storing of fat 

 and the production of red blood corpuscles. The outer layer of the 



2 



FIG. 9. Transverse section of the compact tissue of bone. Magnified 

 150 diameters. From Quain. 



i, Haversian canals ; 2, lacunae ; 3, laminae. 



bone, i.e. the compact bone, is much denser and firmer than the 

 inner layer surrounding the marrow cavity, which is termed can- 

 cellous, but both have the same fundamental structure. The 

 interior of many of the smaller bones, although containing no cavity, 

 is cancellous. 



The matrix of bone is quite impermeable to nutriment, and 

 consequently we find an elaborate arrangement to allow of its 

 distribution. Bone, unlike cartilage, is penetrated by a series of 

 blood vessels running in canals called after their discoverer, the 

 Haversian canals. In a transverse section these appear as circular 

 spaces, often black when viewed under the microscope, owing to 

 their being filled with air or dirt. Around these the bone is arranged 

 in a series of thin concentric sheets, the Haversian lamellae, marked 



