FOOTNOTES: [1] They do, however, take place, since within comparatively few years whole species have completely disappeared; for example, the great auk and the passenger pigeon. In these cases it is not known what part disease played in the destruction. [2] A tissue represents an aggregate of similar cells with the intercellular substances in relation with these as connective tissue, muscular tissue, etc. Where such cell aggregates are localized and where the cells are arranged in structures having definite form and size and performing a definite function, it is customary to designate such structures as organs, as the brain, liver, etc. CHAPTER II NO SHARP LINE OF DEMARKATION BETWEEN HEALTH AND DISEASE.--THE FUNCTIONAL NUTRITIVE AND FORMATIVE ACTIVITIES OF CELLS.--DESTRUCTION AND REPAIR CONSTANT PROCESSES IN LIVING MATTER.--INJURIES TO THE BODY.--THE EFFECT OF HEAT.--THE ACTION OF POISONS.--THE LESIONS OF DISEASE.--REPAIR.--THE LAWS GOVERNING REPAIR.--RELATION OF REPAIR TO COMPLEXITY OF STRUCTURE AND AGE.--THE RESERVE FORCE OF THE BODY.-- COMPENSATORY PROCESSES IN THE BODY.--OLD AGE.--THE DIMINUTION OF RESISTANCE TO THE EFFECT OF THE ENVIRONMENT A PROMINENT FACTOR IN OLD AGE.--DEATH.--HOW BROUGHT ABOUT.--CHANGES IN THE BODY AFTER DEATH.-- THE RECOGNITION OF DEATH. There is no sharp line separating health from disease; changes in the tissues of the same nature, or closely akin to those which are found in disease, are constantly occurring in a state of health. The importance of parasites in causing disease has led to the conception of disease as almost synonymous with parasitism; but it must be remembered that the presence of parasites living at the expense of the body is perfectly consistent with a state of health. Degeneration, decay and parasitism only become disease factors when the conditions produced by them interfere with the life which is the normal or usual for the individual concerned. All the changes which take place in the cells are of great importance in conditions of both health and disease, for life consists in cooerdinated cell activity. The activities of the cells can be divided into those which are nutritive, those which are functional and those which are formative. In the functional activity the cell gives off energy, this loss being made good by the receipt of new energy in the form of nutritive material with which the cell renews itself. In certain cells an exact balance seems to be maintained, but in those cells whose activity is periodic function takes place at the expense of the cell substance, the loss being restored by nutrition during the period of repose. This is shown particularly well in the case of the nerve cells (Fig. 13). Both the functional and nutritive activity can be greatly stimulated, but they must balance; otherwise the condition is that of disease.