ETIOLOGY— CAUSES OF DISEASE 219 



cultural horse on the farm, or the work of the carriage- or saddle-horse 

 with that of the poor man's drudge, to realize that the position which the 

 animal occupies, in other words the occupation as it would be termed in 

 reference to man, exercises a very considerable influence on the suscepti- 

 bility of the system of the horse to certain forms of disease: thus, race- 

 horses are specially liable to sore shins; hacks, hunters, and harness-horses 

 to splints and spavins; and cart-horses to side bones, &c. &c. 



EXCITING CAUSES OF DISEASE 



Those influences which have the power of acting upon the organism in 

 such a way as to excite disease, especially in the system which is already 

 predisposed, were formerly divided into cognizable and non-cognizable: 

 the first section including all agencies the existence of which could be 

 recognized independently of their action in producing disease, such as heat 

 and cold; and the second referring to influences which were not recognizable, 

 but the existence of which was inferred from the effects produced. In this 

 second class all contagious and infectious influences, endemic or epidemic, 

 were included. The system of classification of half a century ago has 

 now become obsolete, but it is worth while to refer to it, because although 

 comparatively recent investigations have proved beyond doubt that the 

 cause of some of the most virulent infectious disorders are quite cognizable, 

 there are other maladies which are endemic, epidemic, and in some cases 

 contagious, which arise from causes the existence of which can only be 

 inferred from the effects produced. The causes which are cognizable or 

 apparent to the senses are mechanical and chemical agencies, food, exertion, 

 excitement, excessive and defective secretion, defective ventilation and 

 drainage, and climatic changes; and it is evident that several of these 

 agencies, which have already been considered, are capable of acting both 

 as predisposing and exciting causes — exciting when they become sufficiently 

 intense to produce the disease to which their primary action only rendered 

 the organism susceptible. Thus, errors in diet, excitement without exertion, 

 excessive secretion, defective secretion, impure atmosphere, and changes 

 of temperature may all act as exciting causes of disease. While they 

 ;uc placed among the predisposing causes — in fact whether the causes are 

 mechanical or chemical in their character — they may be alternately pre- 

 disposing and exciting influences. 



Exciting causes of disease which are cognizable.— 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 



