On to what common structure do both heads of the highlighted muscle insert?.

Of that are from eating black  foods or iron supplements. the most common condition causing  it is a bleeding ulcer.  black poop caused by blood indicate a problem in the upper digestive tract.     along with pain, vomiting, or diarrhea is cause to see a doctor right go see a doctor

Not sure  may be cigarette smoke

This is a true statement.have a nice day!