Take a deep breath.
This is the inside of your lung, seen really closely. At the time his was drawn, we weren’t really sure what it did, other than… breathe.
It’s from a An Examination into the Structure of the Cells of the Human Lungs; with a View to Ascertain the Office They Perform in Respiration, a rather short article (no, really) published a couple of centuries ago in the Philosophical Transactions of the Royal Society of London.