So I was re-reading the Full Metal Alchemist series recently and 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, and I was wondering what is the origin of this trope because 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