372 F. C. MCLEAN AND R. E. ROWLAND 



ject of internal remodeling of compact bone as a topic in the com- 

 parative biology of calcified tissues. 



Neither rock boring nor the excavation of tunnels in compact bone 

 should be regarded solely as a destructive process; both serve to per- 

 form physiologic functions, and can only be understood by considera- 

 tion of the cycles in which they participate. The rock borers and 

 the boring gastropods engage in the removal of calcified tissues in 

 search of either shelter or food, or both. The tunneling in compact 

 bone, by osteoclastic resorption, is unique in that it is followed by 

 reconstruction of new osteons, or haversian systems, which provide 

 a continuing supply of new and reactive bone, meeting the metabolic 

 needs of the organism while leading to maturation and formation of 

 structural bone. 



The internal structure of compact bone has been well described 

 for more than a century; its haversian systems and canals, providing 

 for blood vessels, and its lacunae, housing the osteocytes, with inter- 

 connecting canalicules, are easily demonstrable in ordinary histologic 

 sections. Such sections, however, do not reveal any contrast in the 

 density of the numerous osteons and interstitial lamellae that make 

 up the mass of bone. It has also been known that there is an ex- 

 change of ions between the bones and the circulating fluids of the 

 body, but not until Chievitz and Hevesy, in 1935, exploited radio- 

 active phosphorus, P^-, in the study of bone was the rapiditv of this 

 exchange recognized, and its physiologic significance is still under 

 study, largely by means of tracer elements. 



With the advent of microradiography, and especially when it was 

 employed together with autoradiography, it was found that the bone 

 mineral, instead of being distributed homogeneously in compact 

 bone, exhibits wide variation in the density of the osteons. Figure 1 

 shows a low-power microradiograph of a transverse section through 

 the shaft of the tibia of adult man, in which every gradation from 

 beginning calcification of new osteons to maximum density of ma- 

 ture and fully mineralized osteons appears. Figure 2 illustrates two 

 osteons from the radius of a dog, the same section being viewed 

 at high magnification by four different methods: A, unstained his- 

 tologic section; B, microradiograph; C, autoradiograph, recording 

 alpha tracks from radium-226, administered 2 days before sacrifice; 



