5. The other cavity lies somewhat lower, and extends towards the line of the left nipple, which in fact is where its pulsation is observed. It has a thick surrounding wall, and is hollowed out inside to a cavity like that of a mortar. It is enwrapped and cushioned in the lung, and being surrounded by it, it controls and tempers its own heat. For the lung is both cold in itself and is also cooled by respiration.