Etiology : Causes of Disease. g 



Previous disease in a tissue leaves for a time an impairment of 

 structure and function which may become the essential predis- 

 posing cause of the effective operation of a morbific factor. Me- 

 chanical action on a part may predispose to disease, as for ex- 

 ample, by reducing its circulation and nutrition and thereby di- 

 rectly impairing its power of resistance to other inimical agencies. 

 Not infrequently a pus microbe lies deep in the cuticle or even in 

 the tissues without harm, until there occurs a bruise, or a bony 

 fracture when it at once develops a focus or purulent infection 

 (abcess). 



Exciting Causes are the immediate causes of particular dis- 

 eases, lyike the predisposing causes they may be intrinsic or 

 extrinsic, and the first may be inherent or acquired. 



Among inherent causes are certain of those already named as 

 predisposing causes, but which have come to be forcible enough 

 to develop disease without the intervention of any other observa- 

 ble factor. Thus a hereditary monstrosity (redundancy or de- 

 fect) will appear in successive generations without any apparent 

 additional cause. The appearance of white calves in herds of 

 black cattle, after the whitewashing of their stables shows a 

 similar hereditary operation though the result is not in this case 

 pathological. The birth of blind foals from blind sires or dams, 

 or of foals with distorted feet from mares suffering from severe 

 chronic foot lameness are true pathological sequences, in which 

 the exciting cause is hereditary and operates during intrauterine 

 life. Dentition, as an attendant on early life is often a directly 

 exciting cause, from direct injury by entangled or retained teeth 

 that should have been shed, by fever aroused by the active local 

 changes, or imperfect mastication or insalivation leading to con- 

 sequent indigestion ; in puppies and kittens convulsions are not 

 uncommon as a result. 



Extrinsic Causes are such as operate through the environ- 

 ment. Heat, if excessive and prolonged, relaxes and exhausts 

 the system and exerts a direct influence on the process of sangui- 

 fication so that it may become the direct cause of a variety of 

 diseases. As the result of extensive burns, dangerous congestions 

 of internal organs are liable to occur, and even the prolonged 

 heat of summer often superinduces hepatic and gastric disorder, 

 diarrhoea and dysentery. Fat cattle in uncovered cars or yards 



