Mechanical causes are usually referred to in the first instance under the above heading, and they include all agencies which immediately damage the structures or in any way disturb the functions of an organ or any part of the body. The most intelligible instances of the action of mechanical causes are seen in the effects of blows, sprains, and cuts, which at once produce derangements of parts, described as surgical diseases; but there are other mechanical causes acting with less intensity which may produce affections of the system requiring medicinal treatment - for instance, pressure on any part, although not in many cases productive of injury at the moment, may lead to considerable interference with the freedom of the circulation, and if continued, end by inducing organic disease. The cases in which this result is to be apprehended are numerous enough in the human subject, and the horse is by no means exempt from the consequences of undue pressure from portions of the harness. A notable instance is furnished by excessive and ill-regulated pressure from the saddle, which causes a condition known as wrung withers or galled back or saddle gall, and what is more serious still, the pressure of a badly-fitting collar, which interferes with the circulation of blood in the large vessels of the neck, inducing congestion of the brain, attacks of giddiness, or megrims, and even, under certain circumstances, causing an apoplectic fit.
Pressure on important organs may also be intrinsic in its character as a consequence of the growth of tumours or the deposit of mineral substances, as in the formation of calculi or stones in the bladder or bowels, etc.
Obviously, the pressure which is exerted by the formation of tumours or the deposit of calcareous matter will produce results gradually, and it is also evident that the importance of the disease which is induced will depend entirely upon the function of the part which is affected by the pressure. Tumours or abscesses in the brain, or in the neighbourhood of nerves, or blood-vessels, or on the valves of the heart, in the respiratory passages, the stomach" and intestines, or on, or in, the vicinity of organs the functions of which are essential to life, necessarily cause serious and sometimes even fatal interference with vital processes. Besides the direct mechanical effects of blows or wounds or pressure, there may be immediate depression of the vital powers from the shock to the nervous system reacting on the heart, causing stoppage of the circulation, followed by fainting and sometimes by death.
Chemical causes may act upon the interior or exterior of the body, and they may be either extrinsic or intrinsic. The former will include all kinds of irritants or caustics which may be intentionally or accidentally applied to the surface of the body. The action of these, whether they are powerful acids or caustic alkalis, or other chemicals which have the power of destroying integrity of organic structures, has the advantage of being easily recognized, and if discovered in time, may be considerably checked by the employment of appropriate remedies. The chemical agencies which act as intrinsic causes of disease will include those which have just been referred to, with the addition of all poisonous agents either belonging to the organic or the inorganic classes, and all the deleterious products resulting from deranged digestion or from imperfect oxidation. Of the effete products which are thus converted into animal alkaloids and extractives, the disastrous effects have already been adverted to under the heading of predisposing causes.