After adolescence, after the early twenties, when physical growth has ceased, the ante-pituitary secretion sensitizes the cells of the brain to mental growth.
"The Glands Regulating Personality"
Louis Berman, M.D.
The physical reasons for this are to be found in the increased blood supply consequent upon conscious attention to any part, which sensitizes the nerves of the area and the added number of nerve fibers that are at once put into association with the area by the act of concentration of the attention.
"Health Through Will Power"
James J. Walsh