b PATHOLOGY. 



agents wliich, operating on the body, especially when predis- 

 posed, may excite disease ; amongst them are included those 

 specific causes, or elements of disease, which stamp their in- 

 dividuality on the morbid processes which ensue in the animal 

 body when the germs of such diseases happen to become 

 implanted therein (as in glanders, rabies, rinderpest, scabies, 

 &c.), and are divided into the cognizable and non-cognizable. 

 The first includes all the physical and other agents of whose 

 existence we can take cognizance, independently of their opera- 

 tion in producing disease ; but these so-called non-cognizable 

 causes are now found to consist of certain organisms, the majority 

 of which have been demonstrated to be living matters having 

 specific effects. 



The predisposing and exciting causes of disease, when ex- 

 isting within the system, are called intrinsic, endopathic, or 

 internal; but when they arise without the system, they are 

 denominated extrinsic, exopatldc, or external causes of disease. 



Predisposing causes of diseases. — The most important and 

 generally recognised predisposing causes of disease are — 



(1.) The injiucnce of age. — This is not so striking in the 

 lower animals as in man, but still it plays an important part. 

 In the dog, for example, the period of dentition renders the 

 animal liable to fits of convulsion, paralysis, disturbances of 

 the digestive process, with vomition, irregularity of the faical 

 discharges, weakness, and even inflammation of the eyes, and 

 attendant unthriftiness. Packets is also a disease whicli is 

 only seen during the early period of life, and is witnessed in 

 all our domesticated animals, but more particularly in the dog. 



Canine distemper may be manifested during any period of 

 life : as a great rule, it is only seen during the first few weeks 

 or months of the animal's existence. Again, the strangles of 

 the horse is generally a disease of adolescence. 



It is also well known that the invasion of parasites is much 

 more common during the earlier period of an animal's life ; thus 

 we find the Coenurus cerehralis developed in the brains of sheep 

 and cattle during the first months of life. The Strongylus filaria 

 of the lamb, and its analogue, the Strongylus micrurus of the calf, 

 as a general rule induce disease in these animals in early life. 

 The TricJwncma arcuata, witnessed by me in Icelandic ponies, 

 and referred to in the latter part of this work, are rarely found 

 in horses above two years old. 



