"Pathology is that subdivision of biology which has for its object the study of life in its abnormal relations." It is the science that treats of disease in all its aspects.

By "disease" is meant any condition in which there is a variation from the normal aspect of the organism; it may be either a structural or a functional deviation.

Pathology also may be subdivided into two sections: one known as morbid anatomy and histology, in which the lesions are structural. The other, morbid physiology, in which the changes are functional.

The main heading may be again subdivided into general pathology, that deals with abnormal processes common to the entire organism, such as inflammation, fever, etc., and special pathology, that includes the changes within special organs.

Under etiology are considered the conditions giving rise to disease. They may be either predisposing or exciting.

Predisposing causes are those that in any way lower the vitality of the individual and thus render him more susceptible; such as bad hygienic surroundings, poor food, bad air, noxious gases, fatigue, extremes of temperature, drugs, injury, pre-existing disease.

Exciting causes include mechanical forces, sudden extremes of heat and cold, electricity, poisons, parasites, and also certain mechanical abnormalities, such as defects in the heart-valves.

Although the causes are divided into these two classes, a predisposing cause if acting with great severity may readily excite disease.

The individual may be the seat of two diseases, one acting primarily and another following secondarily.

The latter may be either the direct result of the primary or may have nothing whatever to do with it. Infection of the lung by the tubercle bacillus gives rise to phthisis; later on there may be involvement of pleura or of intestines, or a person suffering from chronic nephritis will often die from a secondary pneumonia.

Traumatism may cause disturbances of function more or less marked according to the extent, severity, rapidity, and duration of its action.

If it takes the form of constant pressure, there will be malnutrition and atrophy of the part on account of the interference with the blood-supply. If the pressure is intermittent, hyperemia may occur and hypertrophy take place.

When the force is sudden the lesions vary according to the instrument used. If sharp, there are lacerations; if dull, contusions.

According to the locality, there may be fractures and concussion. In all these injuries there is greater or less destruction of tissue, followed by the phenomena of inflammation, with either recovery or death.