The brain is part of the nervous system because it sends signals to your body in order for you to move and react. The integumentary protects your body the most from pathogens.
The intended audience is the soldier's wife or girlfriend.
It is clear that the author is currently in the middle of a war, based on the word "foxhole" (a trench that a soldier could hide in) and the fact that he is surrounded by gunfire. It is also clear that he is writing to his wife or girlfriend because he addresses the letter to "my darling."
