156 CHAXGES IN THE HAVERSIAN SYSTEM. 



taken for examination, while others had existed for a 

 considerable period. Some were already undergoing 

 decay, and of others nothing was left. The highly 

 important conclusion was established by these re- 

 searches, that even in fully formed bone, the compact 

 hard tissue was undergoing constant change. Old 

 Haversian rods were continually being removed, and 

 at the very time that adjacent ones were being 

 formed. Thus, in a transverse section of bone, we 

 look upon sections of Haversian rods of every age, 

 and in every degree of change, from the young rod 

 just being formed, to the old one of which the merest 

 traces are to be detected. 



In this way, the firmness and elasticity of bone is 

 efiectually secured, all through adult life, and until 

 we reach old age, when bone ceases to be renewed, 

 and consequently becomes very dry and brittle. In 

 order to form an accurate idea of the wonderfal 

 phenomena taking jilace, we must consider the 

 changes which ensue in a single Haversian system, 

 and which make up its life history — and, that the 

 account may be made as simple as possible, I shall 

 ask you to imagine what would be seen actually going 

 on in an Haversian canal if it were possible for the 

 eye to follow the operations from their commence- 

 ment to their close. 



Let us then consider what we should see were it 

 possible for us to watch the various changes which 

 take place in a single Haversian system after it had 

 reached its mature state, and was about to be re- 

 moved and replaced by another. Just external to the 

 vessel occupying the Haversian canal, in the slight 

 interval between the vascular wall and the surface 

 of the osseous tissiie, we should discern a number of 

 little bioplasts, which would be growing and multiply- 

 ing. Many would be in very close contact with the 

 bony tissue, which is to be gradually acted upon and 

 slowly appropriated by the particles of living matter. 



