12. 
The treatment of cavities, to be euccessful, requires 
considerable training. Thic process will be best under- 
stood by a comparison with the filline of teeth by a den- 
tist. The three important processes in each case are: 
(1) To remove all decay and sterilize to prevent 
further decay, 
(2) To prepare the cavity (Fig. 19) so that it will 
hold the filling. 
(3) To exclude all foreign substances. 
If this work is carefully done the results are much 
petter, even than those of dentistry, for a new tree 
grows up in a few years, surrounding the filling (Figs. 
24 and 25). Since new living layers are annually added 
to the outgside of the wood, the life of the tree, it 
would appear, is only limited by disease or starvation. 
Olid orchards have many trees with weak upright 
crotches, due to poor training in youth. These should be 
braced (Figs. 21 and 22) before the trees are loaded with 
fruit. This may be easily done by tying a cable, or 
chaining to lag-hooks screwed into opposite branches. 
Where three branches can be tied together, forming a 
triangle between the hooks, the greatest strength is se- 
cured. 
A further bracing in the crotch itself (Fig. 21) 
may be done by boring one or more holes through the trunk 
and securely bolting the parts together. The heads of 
