The conception of disease is so inseparably associated with that of life that a brief review of the structure and properties of living things is necessary for the comprehension of the definition which has been given. Living matter is subject to the laws which govern matter, and like matter of any other sort it is composed of atoms and molecules. There is no force inherent in living matter, no vital force independent of and differing from the cosmic forces; the energy which living matter gives off is counterbalanced by the energy which it receives. It undergoes constant change, and there is constant interchange with the environment. The molecules which compose it are constantly undergoing change in their number, kind and arrangement. Atom groups as decomposition products are constantly given off from it, and in return it receives from without other atom groups with which it regenerates its substance or increases in amount. All definitions of life convey this idea of activity. Herbert Spencer says, "Life is the continuous adjustment of internal relations to external conditions." The molecules of the substances forming the living material are large, complex and unstable, and as such they constantly tend to pass from the complex to the simple, from unstable to stable equilibrium. The elementary substances which form living material are known, but it has hitherto not been found possible artificially so to combine these substances that the resulting mass will exhibit those activities which we call the phenomena of life. The distinction between living and nonliving matter is manifest only when the sum of the activities of the living matter is considered; any single phenomenon of the living may appear also in the non-living material. Probably the most distinguishing criterion of living matter is found in its individuality, which undoubtedly depends upon differences in structure, whether physical or chemical, between the different units. Certain conditions are essential for the continued existence of living matter. It must be surrounded by a fluid or semi-fluid medium in order that there may be easy interchange with the environment. It must constantly receive from the outside a supply of energy in the form of food, and substances formed as the result of the intracellular chemical activity must be removed. In the case of many animals it seems as though the necessity of a fluid environment for living matter did not apply, for the superficial cells of the skin have no fluid around them; these cells, however, are dead, and serve merely a mechanical or protective purpose. All the living cells of the skin and all the cells beneath this have fluid around them.