He is both excited and fearful after hearing the witches' prediction about his future because the witches told Macbeth that he would be king, and he is worried about his role in making the prediction come true.
The preposition in question is "on", and its purpose is to show you where the dog chewed. You can remove the preposition to form the sentence "The brown dog chewed the rawhide bone." You can also rearrange it to find the preposition, "on the rawhide bone, the dog chewed" in what is commonly known as the Yoda technique.
It's the first option! This is talking about items being packed in straw!
