In the heart area:
First, five very subtle lights are manifesting : vacuity, light, movement, solidification and continuous flow. They are the qualities or virtues of the five elements.
Then, from those five very subtle lights appear the five elements: space, fire, wind, earth and water.
All the perceptions are based on them. First, the mental body, then the skandhas.
In the brain area:
Then, all the elements are mixed with the external elements (father's semence and mother's blood...) : the resulting physical body is a combination of the five elements constituting the whole of the forms.
Thus, at the brain area, the elements have been mixed with the external elements producing the physical brain.