it, exercise or rest it properly, and the working elements will do their best to keep well or to get well.  What do we do with ailing vegetables?  Dr. Warren, my honored predecessor in this chair, bought a country-place, including half of an old orchard.  A few years afterwards I saw the trees on his side of the fence looking in good health, while those on the other side were scraggy and miserable.  How do you suppose this change was brought about?  By watering them with Fowler’s solution?  By digging in calomel freely about their roots?  Not at all; but by loosening the soil round them, and supplying them with the right kind of food in fitting quantities.

Now a man is not a plant, or, at least, he is a very curious one, for he carries his soil in his stomach, which is a kind—­of portable flower-pot, and he grows round it, instead of out of it.  He has, besides, a singularly complex nutritive apparatus and a nervous system.  But recollect the doctrine already enunciated in the language of Virchow, that an animal, like a tree, is a sum of vital unities, of which the cell is the ultimate element.  Every healthy cell, whether in a vegetable or an animal, necessarily performs its function properly so long as it is supplied with its proper materials and stimuli.  A cell may, it is true, be congenitally defective, in which case disease is, so to speak, its normal state.  But if originally sound and subsequently diseased, there has certainly been some excess, deficiency, or wrong quality in the materials or stimuli applied to it.  You remove this injurious influence and substitute a normal one; remove the baked coal-ashes, for instance, from the roots of a tree, and replace them with loam; take away the salt meat from the patient’s table, and replace it with fresh meat and vegetables, and the cells of the tree or the man return to their duty.

I do not know that we ever apply to a plant any element which is not a natural constituent of the vegetable structure, except perhaps externally, for the accidental purpose of killing parasites.  The whole art of cultivation consists in learning the proper food and conditions of plants, and supplying them.  We give them water, earths, salts of various kinds such as they are made of, with a chance to help themselves to air and light.  The farmer would be laughed at who undertook to manure his fields or his trees with a salt of lead or of arsenic.  These elements are not constituents of healthy plants.  The gardener uses the waste of the arsenic furnaces to kill the weeds in his walks.

If the law of the animal cell, and of the animal organism, which is built up of such cells, is like that of the vegetable, we might expect that we should treat all morbid conditions of any of the vital unities belonging to an animal in the same way, by increasing, diminishing, or changing its natural food or stimuli.