Page:The Principles and Practice of Medicine.djvu/103

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causes of disease, are certain circumstances which precede, and either directly or indirectly produce it. They comprise the various forms of matter, and certain forces or qualities, of which heat, cold, and electricity are familiar examples. The relation between disease and its cause is sometimes very ap- parent, as in inflammation from the presence of a foreign body, the removal of which is equivalent to its cure. At other times the connection is by no means so clear, and has only been ascertained by repeated observations in a large number of instances ; whilst it not unfrequently happens that the action of the cause is so transient, that it altogether escapes observation. Moreover, the real causes are often mixed up with other circum- stances that are in no way operative, and from which it is difficult, and occasionally impossible, to eliminate them.

The causes of disease are intrinsic and extrinsic ; that is, they may be generated or developed in some way or other within the body, or exist external to it. Of the first, the retention in the blood of noxious matters which should have been excreted is an example ; of the second, heat, cold, impure air, improper food, mechanical and chemical influences, and the like, are examples. To enumerate all the causes of disease would be tedious. It may be stated generally, that they include everything which modifies the vital stimuli, i. e., food, air, temperature, light, &c., or interferes with the due regula- tion of the passions and emotions of the mind ; all material agents which inflict mechanical injury upon the textures, and chemical substances which enter into combination with some of their constituents; and lastly, matters whether solid, fluid, or gaseous, which, introduced from without, or formed within the system during natural processes, exert a poisonous influence.