12 PATHOLOGY. 



to ITS under two heads — namely, the intrinsic or endopathic, and 

 the extrinsic or exopathic. The endopathic exciting causes are — 

 d.) Mechanical, which inchide strictures of orifices and tubes, 

 by contraction of their coats, pressure from without, or im- 

 pacted concretions — as stricture, intus-susception, intestinal or 

 urinary calculi ; presence of parasites in the ducts of the liver, in 

 the bronchial tubes and cerebral hemispheres ; the pressure of 

 tumours on the brain and other parts of the body; herniaj; 

 obstructions in the larynx or oesophagus ; nasal polypi, &c. ; and 

 (2.) Chemical causes, which include all retained secretions of 

 excretory organs — such as urea and the other products of the 

 metamorphosis of the nitrogenous constituents of the food and 

 body — occurring as a result of disease of the kidneys, or when 

 nitrogenous products are too abundantly present in the blood, as 

 in azoturia. Again, when from some impediment to the respira- 

 tory function carbonic acid is retained, the animal may die from 

 delirium and coma. If the liver fliils to perform its function, 

 jaundice follows, succeeded by anemia, dropsy, and death. 



Chemical and mechanical causes are also external or exo- 

 pathic, and include all poisonous substances, mineral and vege- 

 table — as acids, caustic alkalies, opium, strychnia, aconite, lead, 

 &c. &c., and all forms of external violence. 



Disease Germs. — Contagia, morbid poisons, and specific disease, 

 virus are now included under exopathic causes of disease, 

 although the evidence is not very clear upon all points, but 

 tliey are generally propagated by infection, contagion, and 

 inoculation (see Origin of Contagious and Infectious Diseases). 

 There is yet some evidence of their endopathic origin. 



Exopathic predisposing and exciting causes are those due to 

 the influence of climate, food and water, ventilation and drainage, 

 locality, geological formation, weather and seasons, malaria, 

 germs, work, and the want of it. 



Of the influence of climate, the diseases named kumree and 

 bursatee, common in the East, afford well-known examples. 



Food and water. — The solid and liquid ingesta are fertile 

 sources of disease — \st, by being insufficient or ill-proportioned 

 in quality ; 2d, by being deficient or excessive in quantity. 



Unsuitability of food, either in consequence of a deficiency, 

 over-abundance, or improper combination of nutritive con- 

 stituents, is a very frequent cause of disease. This has been 



