So I was re-reading the Fullmetal Alchemist series recently. I noticed this scene below with the man presumably on the verge of death with I presume to be his "soul" coming out of his mouth. I was wondering what the origin of this trope is, since I have often seen this in other mangas and animes.

![Full Metal Alchemist Pg. 151  Vol. 1][1]


  [1]: https://i.sstatic.net/ckrtd.png