Disease and Its Causes | Page 3

of disease is inseparable from the idea of life, since only a living thing can become diseased. In any dead body there has been a pre?xisting disease or injury, and, in consequence of the change produced, that particular form of activity which constitutes life has ceased. Changes such as putrefaction take place in the dead body, but they are changes which would take place in any mass similarly constituted, and are not influenced by the fact that the mass was once living. Disease may also be thought of as the negation of the normal. There is, however, in living things no definite type for the normal. An ideal normal type may be constructed by taking the average of a large number of individuals; but any single individual of the group will, to a greater or less extent, depart from it. No two individuals have been found in whom all the Bertillon measurements agree. Disease has reference to the individual; conditions which in one individual would be regarded as disease need not be so regarded in another. Comparisons between health and disease, the normal and the abnormal, must be made not between the ideal normal and abnormal, but between what constitutes the normal or usual and the abnormal in a particular individual.
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.
Living matter occurs always in the form of small masses called "cells," which are the living units. The cells vary in form, structure and size, some being so large that they can be seen with the naked eye, while others are so small that they cannot be distinctly seen with the highest power of the microscope. The living thing or organism may be composed of a single cell or, in the case of the higher animals and plants, may be formed of great numbers of cells, those of a similar character being combined in masses to form organs such as the liver and brain.
In each cell there is a differentiated area constituting a special structure, the nucleus, which contains a peculiar material called "chromatin." The nucleus has chiefly to do with the multiplication of the cell and contains the factors which determine heredity. The mass outside of the nucleus is termed "cytoplasm," and this may be homogeneous