but only gradually can this guidance become self-control. This necessity is seen more clearly as we continue our physiological inquiry. The preceding considerations refer chiefly to the slow processes by which the various parts of the body must be built up step by step, under the guidance of outside intelligence, which furnishes the proper conditions of physical growth. Equally certain, and within the legitimate scope of true physiology, is the influence which the mind of the individual exercises upon the growth of the body. This difficult half of the subject presents itself in increasing importance as science advances. The particular theory of mind held by individuals does not affect our inquiry. Everyone understands the term, and gives to its influence a certain importance. Our perception of the degree of power exercised by the mind over the body, and the importance of that power, will continually grow as we observe the facts around us. It is a fact of every-day experience, that fright will make the heart beat, that anxiety will disturb digestion, that sorrow will depress all the vital functions, whilst happiness will strengthen them. How often does the physician see the languid, ailing invalid converted from mental causes--through happiness--into a bright, active being! Medical records are full of accumulated facts showing the extent to which such mental or emotional influence may go; how the infant has been killed when the mother has nursed it during a fit of passion, or the hair turned gray in a single night, through grief or fright. We find that the mind, acting through the nervous system, affects not only the senses and muscles--the organs of animal life, under the direct influence of the cerebro-spinal axis--but that it may also extend its influence to those processes of nutrition and secretion which belong to the vegetative life of the body. Emotion can act where Will is powerless, but a strong Will also can acquire a remarkable power over the body. It has been remarked ‘that men who know that there is any hereditary disease in their family, can contribute to the development of that disease, by closely directing their attention to it, and so throwing their nervous energy in that direction.’ It was a remark of John Hunter ‘that he could direct a sensation to any part of his body.’