It is found by dissection that the brain of an intellectual man exhibits a larger number of convolutions than one of small intellectual calibre, and that the convolutions are deeper and the layer of gray substance thicker, and in consequence of the increase in number and depth of convolutions there is a wider expanse of surface as well, for the distribution of gray matter. Hence the relative proportion of gray matter in different brains has come to be regarded by physiologists as a test of mental power. Many idiots have large and well formed brains but the convolutions are shallow and few and the gray matter small in quantity and extent of surface. Physicians often ask me how I can estimate the relative quantity of gray matter in a living head without cutting into it. I refer them to the study of quality and temperament which I have clearly expounded in this lecture. Do you ever find hickory leaves growing on a pine tree? Show me the bark of a tree and I'll tell you the quality of the wood within; show me the skin, the hair, the eyes of a man and I'll tell you the quality of every organ in his body as well as the quality of the brain. I recently astonished the superintendent of an insane asylum by pointing out to him that the quality of the hair, the eyes and the skin of idiots was essentially different from the quality of those of more highly endowed persons, and could be told in the dark by a person of educated sensibilities. The quality and texture of the brain being determined, the next step is the consideration of its size. Other things being equal in all natural objects, size is the measure of power. By the term "other things" in relation to the brain, we mean temperament, quality and health. This simple principle explains why a great many people who carry large heads are endowed with but little intellectual power. Their heads are filled with "sawdust," in other words, a brain of poor quality, supported by a feeble body, or vitiated by excessive temperamental conditions. Men who carry small and misshapen heads are often brilliant in certain directions, and this limited brilliancy in special lines causes them to be spoken of by superficial observers as men of great ability and apparent exceptions to the phrenological rule. The fact remains, however, that in no case is comprehensive greatness ever exhibited in a head of small dimensions. [Illustration: Small Head. Brilliant in Observation, Deficient in Reflection.] Large size of brain, accompanied with robust health, high quality and good temperamental conditions, gives the highest phase of powerful mentality and comprehensive greatness. Small size of brain, with poor health, low quality and erratic temperamental conditions gives the lowest form of mentality and constitutional inferiority. Between these two extremes we may find every conceivable modification and form of human character according to the various combinations of normal and abnormal conditions.