ANIMAL GRAFTS AND REGENERATIONS. 93% 
the middle, and, on examining the organ some months after- 
ward, he observed complete reunion between the separated 
parts, and ascertained that the solution of continuity was 
filled up by a fresh growth of muscular tissue. Thus all 
the tissues of the animal system can be reproduced in the 
grown-up subject, and these regenerations are uniformly 
identical operations with those which have as their result 
the first formation and the development of the very same 
tissues in the embryo or in the young animal. 
In the practice of the surgical art the knowledge of 
these facts of reproduction has been the occasion of more 
or less remarkable inventions and operative methods, some 
of which are as yet under investigation. ‘Those which re- 
late to the renovation of bony tissue have interested the 
public peculiarly of late years. It has always been known 
that, when a bone is broken, the solution of continuity in 
it is filled up, after a certain time, by a portion of bone of 
fresh formation, a true bony scar, a callus. It was not be- 
fore the middle of the last century that a French physiolo- 
gist, Duhamel, and after him a Neapolitan physician settled 
at Paris, Troja, investigating the phenomenon of callus 
closely, discovered its physiological mechanism. They 
believed they could observe that the chief agent in the 
elaboration of bone is a thin fibrous sheath applied and 
adhering closely all around the bones, the membrane called 
the periosteum.’ Their experiments were neither numerous 
nor striking enough to disclose to surgeons the advantage 
that might be gained from the knowledge of the bone- 
making property peculiar to the periosteum. ‘The atten- 
tion of practitioners did not begin to be drawn to this point 
until later, toward 1830, by the labors of a professor at 
1 The bones may be regarded as formed of three concentric layers, 
each inner one sheathed in another—the inmost one the marrow, next 
the bony substance properly so called, which is covered by the perios- 
teum. 
