PATHOLOGY rdeor, disease,' and Abyss, a discourse ') is the science of diseases, and especially of those which affect men and animals which are the subjects of medical treatment.
For the perfect knowledge of the nature of a disease, the first circumstance to be determined is its cause ; and this is commonly regarded as twofold. The predisposing cause or condition (for the term cause cannot fairly be used in this sense) ie that state of any individual which renders him peculiarly liable to the attack of any or of some particular disease, of which another person or he himself at another time might be in little danger. Of these predisposing con ditions the most important is hereditary disposition, by which an individual being constructed with the same iSeculiarities of internal and of external form and composition, which one or both of his parents possessed, is liable to the same diseases as they were. Such are the peculiarities of temperament or constitution with which each indivi dual in born, and by which he is through life disposed to a peculiar character of disease ; and such also are the special hereditary disposi tions to scrofula, gout, insanity, some forms of asthma, and probably many more diseases. Other predisposing conditions are the peculiarities of constitution which are acquired in the course of life by particular modes of living. Each individual is adapted by habit and other cir cumstances to the conditions of climate, &c. in which he is placed, and is peculiarly liable to be affected by changes of these external conditions. A person of effeminate habits living carefully secluded from all excitants of diseases, is much more liable to be affected by exposure to any of them than one whose frame by a hardy course of life is rendered comparatively invulnerable to all. Any means by which the strength of the body is reduced render it more liable to diseases of Ali kinds, and hence our idea of bodily strength is drawn not more from the muscular power of the individual than from his immunity from the effects of those circumstances which in others excite disease. There are also local peculiarities of individual organs of the body which render them especially liable to disease ; such are the state bordering upon disease which is brought on by constant over excitement of any organ ; the condition of an organ which has once been affected with a disease, and which is commonly thenceforward particularly liable to a repetition of it; the state of the organs which at different periods of life renders one more than another liable to disease, so that the same excitant will he most likely to produce in the child an affection of the head, in the youth a disease of the chest, and In the adult or old person some disorder of the abdominal organs.
Any of these predispositions however may exist throughout life without the occurrence of actual disease ; in order to produce disease, some more immediate or exciting cause Is necessary. This excitant must be the more powerful the lees the predisposition : but under whatever circumstances disease is produced, the predisposing condition of the patient may be expected to confer upon it a corresponding peculiarity of character. The exciting causes of diseases are any change* of a certain extent in the conditions of the external circum stances in which man is placed. For example, a certain range of external temperature, a certain constitution of the atmosphere, a certain supply of pure food and drink, a certain amount of mental and bodily exertion, are circumstances essential to health, and alterations in any of them may produce dine ac, of which the nature and the seat will be determined in part by the predisposition of each individual, and In part by the peculiar mode of action of the excitant. Thus, after the same exciting cause (for example, exposure to oold and damp), one person may have rheumatism, another pleurisy, a third ophthalmia, and a fourth may escape altogether unharmed. But there are other excitants of disease which prevail over all predisposition, and produce a certain character of disease, which the constitution of the patient can only slightly modify ; each are the materials of all contagious and epideinle disorders, as influenza, small-pox, measles, &e., which produce in all whom they attack a similar affection. Many persons however escape from the effects of these excitants, and by long exposure become inured to them ; hence the diseases of peculiar climates (endemics) affect foreigners much more than natives ; but even in those persons in whom they do not produce disease, these conditions, which are excitants of disease in others, modify the characters of diseases that occur from any other source ; and hence in the course of an epidemic all diseases have a tendency to assume some of the characters of that which is prevalent. Other excitants of disease, still more universal in their influence and more constant in their consequences, are all things which act immediately on the composition or construction of the body or of the blood, such as mechanical and chemical injuries, including poisons of all kinds.