When a spirit reincarnates, it is through the perispirit that it is united with the physical body. This union is carried out during the gestation period, where it is first tied to the embryo through a fluidic bond or "cord", one that is actually an extension of the perispirit, that attracts it with an irresistible force, from the moment of conception.  As the embryo develops, the fluidic cord shortens, and the perispirit is joined, molecule by molecule, to the embryo.

At this time, the embryo also receives a kind of "glue", the vital principle, which, as we mentioned earlier, establishes a necessary tie between the fluidic material of the perispirit and the dense material of the physical body, and thereby brings life to the physical organism. The perispirit then binds to the biologic system as it forms, little by little, consolidating the most subtle ties, until the point at which it begins to animate the physical body.  The animation is carried out through an energy, "more or less vigorous, that will determine the total possible longevity of the individual", taking into account the limits of genetics. 

In other words, the vital force is impacted by the modifying actions of the genes inherited from the parents, which transmit organic dispositions to the physical body¹.  Through the process of this union, the action of the vital force is what leads the perispirit to develop its functional properties. 

The perispirit employs some very important functions in its relationship with the soul or spirit.  To follow are some of the most significant.  Note that often times, these roles are quite interrelated.