Nature of Disease. Normal activities of tissues concerned with self-preservation; in disease their efficiency impaired. Causation of Disease. - Action of external forces nearly always present, but powers of resistance in the tissues an important element. A. Influence of external forces: I. Direct action of (1) the physical forces, (2) poisons and (3) infective agents; II. Indirect action in producing susceptibility to disease. B. Influence of internal conditions: I. Congenital diseases and susceptibilities; II. Influence of inheritance (1) on racial and individual characters, (2) on anatomical or morphological and physiological abnormalities, (3) on susceptibilities and predispositions to disease, (4) on the constitution of the body as a whole and its individual parts. (5) Theories of heredity; III. Influence of age and sex. Terminations of Disease. - Recovery implies vis medicatriz naturae. Death by failure of respiration or of heart's action.
PATHOLOGY, as we have seen, is concerned with morbid conditions, taking account of the causes of disease, the alterations in the actions, and the changes in the structure of the tissues involved. This implies a knowledge of the condition of the body-in health and a comparison between the healthy and morbid states. In this preliminary section it will be proper to lay down some general principles in regard to the difference between normal and pathological actions.
In our conception of the living body we are not to look upon it merely as a whole, but also in its finer elements. John Hunter laid it down as one of his most important primary principles that "every individual particle of animal matter is possessed of life, and the least imaginable part is as much alive as the whole." This principle of John Hunter's may be regarded as the basis of the modern Cell theory, which, in the hands of Goodsir, Virchow, and others, has been applied to Pathology. According to this theory, the tissues of the body contain multitudes of quasi-independent bodies, which have their own actions and their own life. We do not properly appreciate any action of the body, whether normal or pathological, unless we take into account the finest details of the tissues and have regard to the active cell-life which goes on there.
The primary actions of the cells will be found to be remarkably similar to those of the larger structures of the body and of the body as a whole. Each of the cells is engaged, in the first instance, in self-preservation. It has its own substance to preserve, and it has also in many cases a certain portion of intercellular substance to look after. Its first duty is to resist any disintegrating action and to make up for any waste which may occur in the exercise of its ordinary functions. In like manner the body as a whole has, for its primary action, the preservation of its own existence, opposing its resistance to forces external to itself, and making up for the ordinary waste of life. To this are subservient all the constant organic functions, digestion, respiration, circulation, etc. The individual organs of the body have also, in many cases, special arrangements for self-preservation, which come into play when any interference from without incommodes them in their functions. Thus resistance to forces external to themselves in order to self-preservation enters into the primary actions of all living animal structures. This may be illustrated before going further.
A living egg consists of a single cell, surrounded by a mass of nutritious matter destined to afford material out of which the future animal is to be built up. But the egg, by virtue of the living principle in it, possesses, as John Hunter showed, a remarkable power of resisting external influences. It resists the influence of cold and of the agents of putrefaction to a degree which contrasts very strikingly with an egg which is not alive. When a dead egg (e.g., one that has been killed by previous freezing) is placed in a freezing mixture its temperature soon falls to 32° F., and it begins to swell and congeal. In the case of a fresh egg, however, the temperature falls to 29 1/2° without freezing, and it takes 25 minutes longer than the dead one to freeze. Before freezing, the temperature rises to 32°. Again, all through the process of hatching, the yolk, and what remains of the albumen, continue perfectly sweet; but if a dead egg be in the nest it becomes putrid like an ordinary piece of dead matter under similar circumstances. These observations of John Hunter show that a small piece of living matter has a special power of resisting the influences of external forces such as dead matter does not possess.
The body as a whole is also engaged in resisting the action of forces external to itself. An example of this is afforded by the preservation of the bodily temperature in warm-blooded animals. In such animals there is a continuous loss of heat by radiation and convection, and it is an important function of the body to compensate for the loss. Again, locomotion and even the retention of the erect posture are effected by the opposition of the various organs of locomotion to the action of gravitation. The resistance to this force involves the functions of bones, ligaments, muscles, and nervous system.
In the actual living body, then, the cells are engaged in preserving themselves from influences which may exist inside the body but are external to themselves, the regular wear and tear of life, and the body as a whole is similarly engaged. Health is preserved so long as the individual elements and the tissues as a whole are completely successful in resisting all disturbing influences, so long as they preserve themselves in a perfect state. In that case all the functions are performed with such ease that they do not obtrude themselves on the consciousness, and the body as a whole is preserved from decay and injury. On the other hand, when the forces external to the body as a whole or to its individual elements successfully obtrude themselves and the due efficiency of the living structures is impaired, then we have a state of disease. In disease, therefore, we have to take into account the vital activity of the tissues, chiefly of the cellular elements, and the alterations wrought by forces external to the structures concerned.