The line of Head indicates the mental stamina, brain quality, ability to make decisions and major upsets or mental condition during the complete life.
The Line of Head includes in its scope not only those of mental faculties, but that of mental intensity, creativity (or lack thereof), the manner of talent possessed, and the lifespan of an individual. It functions in union with the line of life--one physical, one mental--to ascribe the overall health of of the bearer. For one can live a long life whilst wanting in spirit, surviving in a vegetative existence requiring the efforts of others; likewise may one's constitution flag while their mind remains as keen and well-honed as ever.
In addition to its current mental significance, it is an important indicator towards the potential of the bearer when both hands are consulted. Perhaps the bearer has waxed indolent in a relaxing of academic standards, or perhaps they never would have had the potential to begin with: it is all related, dispelled, and revealed in the Line of Head.