what is an extrinsic cause of disease
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Health consists in a natural and proper condition and proportion in the functions and structures of the several parts of which the body is composed. The standard of health is not, however, the same in all individuals; that which may be health to one may be disease to another. Thus: the healthy pulse in adults averages from 70 to 80; yet there are some in whom 90 or 100 is a healthy pulse. Some persons fatten on a quantity of food on which others would starve. The animal functions, muscular strength and activity, nervous sensibility, and the sensorial powers, vary still more in different individuals, yet all within the limits of health.
Causes of disease are those circumstances which essentially precede it, and to the operation of which its occurrence is due. In many instances these circumstances elude our observation. In many others, the true cause, if apparent, is combined with other antecedent circumstances which have no share in producing the disease, and yet are liable to be mistaken for causes. These circumstances are to be sifted, and the true cause discovered, only by the attentive observation of large numbers of cases in which disease is produced. Thus, it was long a matter of doubt whether the Itch could be engendered from filth, as well as from contagion; but, since microscopic investigation has discovered the existence of the Itch-mite, no doubt remains that this insect is the only essential cause of the disease.
The causes or circumstances inducing disease may be intrinsic, or existing within the body; or they may be extrinsic, having their origin without the body. Extrinsic causes are very numerous; comprising all the agencies which can act upon the body or mind, such as temperature, air, moisture, food, poisons, mechanical and chemical influences, sensual impressions, etc, etc.
But the common causes of disease are seldom of a decided and positive character; they are often present without disease ensuing and they are known to be causes only because disease is observed to ensue in a greater number of cases when they are present than when they are absent. Thus, improper food is a cause of indigestion, and exposure to cold is a cause of catarrh; yet many persons eat unwholesome food without suffering from indigestion, and many are exposed to cold without taking cold. But those who do suffer from indigestion observe that they do so more after taking improper food; and those who are afflicted with catarrh can often trace it to exposure to cold. In some cases, however, where the predisposition to disease is sufficiently strong, it may, under certain circumstances, become in itself a sufficient cause of disease; thus, a person with a very weak stomach always has indigestion. So, likewise, exciting causes, if sufficiently strong, may produce disease without predisposition; thus, a person not predisposed to indigestion, may be pretty sure to get it, if he takes a sufficient quantity of fat, raw cucumber, or any such indigestible matter. Take another example. A healthy person, living in a marshy district, may not get Ague until he becomes debilitated from any cause, such as cold or fatigue; then the poison will act. But, without his being thus weakened, if the exciting cause be made stronger by his sleeping on the very marshy ground itself, then the poison may act without predisposition.
Causes of disease are those circumstances which essentially precede it, and to the operation of which its occurrence is due. In many instances these circumstances elude our observation. In many others, the true cause, if apparent, is combined with other antecedent circumstances which have no share in producing the disease, and yet are liable to be mistaken for causes. These circumstances are to be sifted, and the true cause discovered, only by the attentive observation of large numbers of cases in which disease is produced. Thus, it was long a matter of doubt whether the Itch could be engendered from filth, as well as from contagion; but, since microscopic investigation has discovered the existence of the Itch-mite, no doubt remains that this insect is the only essential cause of the disease.
The causes or circumstances inducing disease may be intrinsic, or existing within the body; or they may be extrinsic, having their origin without the body. Extrinsic causes are very numerous; comprising all the agencies which can act upon the body or mind, such as temperature, air, moisture, food, poisons, mechanical and chemical influences, sensual impressions, etc, etc.
But the common causes of disease are seldom of a decided and positive character; they are often present without disease ensuing and they are known to be causes only because disease is observed to ensue in a greater number of cases when they are present than when they are absent. Thus, improper food is a cause of indigestion, and exposure to cold is a cause of catarrh; yet many persons eat unwholesome food without suffering from indigestion, and many are exposed to cold without taking cold. But those who do suffer from indigestion observe that they do so more after taking improper food; and those who are afflicted with catarrh can often trace it to exposure to cold. In some cases, however, where the predisposition to disease is sufficiently strong, it may, under certain circumstances, become in itself a sufficient cause of disease; thus, a person with a very weak stomach always has indigestion. So, likewise, exciting causes, if sufficiently strong, may produce disease without predisposition; thus, a person not predisposed to indigestion, may be pretty sure to get it, if he takes a sufficient quantity of fat, raw cucumber, or any such indigestible matter. Take another example. A healthy person, living in a marshy district, may not get Ague until he becomes debilitated from any cause, such as cold or fatigue; then the poison will act. But, without his being thus weakened, if the exciting cause be made stronger by his sleeping on the very marshy ground itself, then the poison may act without predisposition.
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