Imsety (Jmstj) the human headed son of Horus, protected the liver of the deceased and was in turn protected by the goddess Isis.  His role was to make sure that the dead would make it to the afterlife, as he is asked to lift them up by Horus: “You have come to N; betake yourself beneath him and lift him up, do not be far from him, (even) N, in your name of Imsety.”  

Just as Osiris (mortal) and Horus (immortal) are recognized as two names for the same being, Imsety in Egypt is also known as Thoth.

To stand up meant to be active and thus alive while to be prone signified death. In Spell 151 of the Book of the Dead Imsety is given the following words to say: “I am your son, Osiris, I have come to be your protection. I have strengthened your house enduringly. As Ptah decreed in accordance with what Ra himself decrees.” Again the theme of making alive and revivifying is alluded to through the metaphor of making his house flourish. He does this with the authority of two creator gods Ptah and Ra (or Re).

Spell 148 in the Book of the Dead directly associates all four of Horus’s sons to the four cardinal points. Imsety was associated with the south.

The four sons of Horus were born from a lotus flower (see Maya’s dream) and were solar gods associated with the creation. It was believed that Anubis gave them the funerary duties of mummification, the Opening of the Mouth, the burial of Osiris (the mortal incarnation of Horus) and all men. Horus later made them protectors of the four cardinal points (north, south, east and west). In the Hall of Ma’at they sat on a lotus flower in front of Osiris. Most commonly, however, they were remembered as the protectors of the internal organs of the deceased. Each son protected an organ, and each son was protected by a goddess. 
Source: Egyptian Myths