GENERAL METHODS 



123 



siderable depth. Half an inch is all that can 

 usually be counted on. 



Aside from the necessity of making sure that 

 all infested wood is removed, it does not make 

 much difference in what shape the interior of the 

 cavity is left. It does not have to be smooth, ex- 

 cept in so far as may be required by the nature of 

 the dressing to be used. Tar, for instance, will 

 not flow behind large semi-detached chips, but as- 

 phalt may cover them easily. 



SHAPING THE MOUTH 



The excavation finished, the next step is the 

 proper outlining and edging of the opening. The 

 mouth of the cavity 

 must be so shaped that 

 the filling will be re- 

 tained, the formation 

 and growth of calluses 

 facilitated, and the en- 

 trance of water into 

 the cavity made unlikely. 

 The first purpose is at- 

 tained by making the 

 cavity larger back of the 

 mouth than it is just at 

 the mouth. The entrance of water is made less 

 easy by slanting upward the upper and lower 

 edges of the opening, so as to form water-sheds. 



Vertical section showing top 

 and bottom of cavity slant- 

 ed up to prevent entrance 

 of water 



If the top of the incision is 

 made too square callus 

 growth does not start from 

 the top 



