STRUCTURE AND DEVELOPMENT OF BONE. 
337 
distinctness of outline and merges into the surrounding gum, from which it can no 
longer be distinguished. Here then we have a second instance in which an ab- 
sorbent tissue has its function finally suspended, and a new one substituted. In the 
place of increasing at the cost of pre-existing tissue, growth is suspended, and it 
unites with and assumes a similar office to that of the surrounding gum. 
In conclusion, the authors may allude to an interesting fact, with reference to the 
subject of absorption, which has been recently observed by Mr. Stanley and subse- 
quently by Mr. Bowman. In the treatment of ununited fracture, the practice has of 
late been successfully adopted of drilling cylindrical holes into the bone in the neigh- 
bourhood of the injury, and of driving in pegs of ivory, accurately fitted to the per- 
forations, for the purpose of setting up action in their vicinity. These pegs, after 
remaining in the bone several weeks, have been removed, and have been found on 
inspection to present erosions similar to those which are seen on absorbing bone or 
tooth. In fact, they closely resemble the fangs of temporary teeth recently attacked 
by absorption. Mr. Bowman kindly placed at the disposal of the authors one of the 
pegs which had been used for this purpose, a section of which presents, at the parts 
eroded, in its microscopic characters, the usual emarginated outline so frequently 
alluded to in the foregoing pages. They have not however had the opportunity of 
investigating the condition of the tissues around the pegs ; but it is exceedingly 
probable that absorption has in these cases been effected by processes similar to those 
which occur in the formation of Haversian spaces. 
Explanation of the Plates. 
PLATE VI. 
Fig. 1. Transverse section of compact bone, showing the ordinary appearances. 
a. Haversian system, h. Interstitial laminae, c. A new Haversian system 
within an older one, the Haversian canal obliterated by the development 
of a lacunal cell. 
Fig. 2. Transverse section of compact bone, showing an Haversian space, with its 
characteristic emarginated outline. 
Fig. 3. The same, from a less compact part of the bone, from a man aged fifty-six. 
Fig. 4. Transverse section from the fibula of a child two years old, showing the 
general characters of bone in young subjects, a. Haversian space. 
Fig. 5. Section of compact bone, showing several Haversian systems enclosed within 
a single series of laminae, a. A lacunal cell attached to the surface of an 
Haversian canal. 
Fig. 6. Section showing the characters of a new Haversian system, developed within 
an older one. 
