220 HEALTH AND DISEASE 



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 con- 

 siderable 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, &c. 



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 re- 

 acting 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 



