A very common idea, but one founded on no certain or feasible grounds, is that which maintains that our bodies undergo a complete change and renewal of all their parts every seven years. The ‘mystical’ nature of the number seven, has had an unquestionable effect in originating this opinion; and although the age of fourteen and again that of twenty-one may be regarded as marking the attainment of youth and manhood or womanhood respectively, yet physiology gives no countenance to the popular opinion that of necessity these periods are those of sweeping bodily change. On the contrary, it might be shewn that the periods at which full growth of body is attained vary with climate, race, and constitution—that is, with the personal nature, and with the physical surroundings of individuals, communities, and nations. The true state of matters as disclosed by physiology, leads us to contemplate actions and changes which are of infinitely more wondrous kind than those involved in the idea of septennial change. For if there is one axiom which physiology maintains more constantly than another, it is that which teaches that constant and _never-ceasing_ change is the lot of life from its beginning to its end.
No part of the body of a living being is free from these changes of substance, through which indeed every act of life is carried on. Every movement of a muscle—the winking of an eyelid or the lifting of a finger—implies waste of the organs and parts which move. The thinking of a thought implies wear and tear of the organ which thinks—the brain itself. Were it possible to spend existence even in a perfectly still and rigid condition, there are still actions to be performed which are necessary for the maintenance of life, and which necessitate continual waste and wear of the tissues. Thus the beating of the heart, the movements of our chest in breathing, and the very act of receiving and digesting food—actions which are in themselves concerned with the repair of the frame—can only be performed through the intervention of processes of work, and waste of body. So that a living being is to be regarded as passing its existence in a constant state of change. Its particles are being continually wasted, and as incessantly renewed; and although the growth of our bodies may be said to culminate at various periods of our life, yet it is anything but correct to say that there are marked epochs of change in human existence. The truth is that change and alteration are our continual heritage; and it is strange indeed to think that not an organ or part of our bodies exists which has not repeatedly in its history been insensibly and gradually, but none the less perfectly, renewed in all its parts. Our particles and substance are being dissipated in very many ways and fashions. Chemically and physically, we are in a state of continual break-down; whilst on the other hand, it may be shewn that the forces of life are enlisted powerfully on the side of renewal and repair.