Dr. Brown-Sequard, who strongly favours the idea of superiority, both in size and weight, of the left over the right cerebral hemisphere, also ascribes the source of this to the greater frequency and energy of all right-hand movements. He reverts to an argument derived from left-handedness when discussing his theory that the two hemispheres practically constitute two distinct brains, each sufficient in itself for the full performance of nearly all mental operations; though each has also its own special functions, among which is the control over the movements and the organs of opposite sides of the body. “Every organ,” he says, “which is put in use for a certain function gets developed, and more apt or ready to perform that function. Indeed, the brain shows this in point of mere size; for the left side of the brain, which is used most, is larger than the right side. The left side of the brain also receives a great deal more blood than the right side, because its action preponderates; and every organ that acts much receives more blood.” He accordingly affirms that the growth of the brain up to forty years of age, if not indeed to a considerably later period of life, is sufficiently marked to require the continued enlargement of the hat. The evidence he adduces, based on observing that a hat laid aside for a time and then resumed, always proved to be too small, is probably deceptive. But the growth of the adult brain is no longer disputed. It was indeed affirmed by earlier physiologists, as by Sœmmering, the Wenzels, and Tiedemann, that the brain attained its greatest development not later than seven or eight years of age. But this idea is now entirely abandoned; and--without going so far as to affirm with Dr. Brown-Sequard, who claims that at the age of fifty-seven he found by the test of hat measurement that his head had enlarged within every six months,--the latest observers adduce proof of continuous increase of brain weight, if not of bulk, until the greatest average weight is reached between thirty and forty years of age.