Sentence:  A <WSD>percussor</WSD> must be in a mental attitude toward the examined subject that is twofold in character; he knows, or should know, beforehand that there will be certain phenomena of sound that he is sure to discover, and that there are certain other phenomena that it is possible to discover, and that await his intelligent elicitation. 
Meaning of the word marked with <WSD> tags:  A physician who provides light blow to a body part as a diagnostic technique.