CAUSES OF DISEASE. 7 



The bones are also much more liable to disease during youth 

 than middle age ; for example, osteo-porosis is rarely witnessed in 

 an animal which has passed its fifth year. I say rarely witnessed, 

 for I have seen some horses affected with this disease which had 

 passed the age of seven years ; but even here I was induced to 

 think that the morbid changes had probably originated during 

 the earlier years of the affected animals' lives. 



Again, we find that disease of the facial bones is generally seen 

 in young horses, and is doubtless closely connected with the 

 process of dentition. IN'or must we forget to mention that denti- 

 tion is not always free from attendant ill conseciuences ; in some 

 cases the crowns of the temporary molars are not shed, but 

 become entangled in the newly cut permanent ones, causing 

 difficulty in mastication, indigestion, and unthriftiness, more 

 particularly in horned cattle ; whilst in the horse, at about the 

 age of four years, a true dental cough results from irritation 

 induced by the cutting of the sixtli molar tooth. 



It becomes a matter of speculation whether those causes of 

 lameness which are so commonly met with in young horses are 

 results of predispositions or not. When it is taken into con- 

 sideration that the bones of the young are more vascular 

 or succulent, contain less earthy matter than those which 

 have arrived at full maturity, and that these young bones are 

 subjected to inordinate work, concussion, and alteration of in- 

 cidence by erroneous shoeing, we must conclude bhat there is not 

 always a predisposition to disease, but that disease is induced by 

 subjecting the animal to work which its physiological condition 

 and strength are unable to withstand. At the same time, many 

 animals develop diseases of the bones and joints without the 

 aid of exciting causes, which leads us to conclude that in them 

 there is at this period of life a predisposition to disease. 



Both foals, calves, and lambs are very early — indeed often 

 within the first few days of their existence — subject to disease of 

 their articulations, very frequently without any apparent cause. 

 Here we may safely conclude that great vascularity of the ex- 

 tremities of the bones constitutes a predisposing cause of disease. 



During the middle period of life, animals as a rule have few 

 special tendencies to disease. Such affections as navicular arthritis, 

 and those conditions of the respiratory track which give rise to 

 roaring, whistling, &c., are more frequently witnessed during the 



