TUBERCULOSIS OF THE HORSE 51 



It is now admitted that all these affections may properly be classed among 

 the true contagia, depending as they do on the introduction of a specific 

 organism from without. 



Experimental evidence leaves no room for doubt that the affection 

 occurs by introduction of the germs into the respiratory organs along with 

 inhaled air, and also into the digestive organs with the food. 



In the horse, as in most animals, the lungs are more often affected than 

 other organs, and it may be assumed that the most frequent mode of 

 infection is through the respiratory passages. The animal, however, is not 

 exempt from the other mode of infection. 



Inoculation with tuberculous matter 

 causes, in most cases, tuberculosis by the 

 introduction of the virus into the blood. 



Symptoms. — Even in cases where 

 signs of disease are present there is 

 nothing to be discovered which is posi- 

 tively indicative of tuberculous de2Josit 

 — cough, difficulty of breathing under 

 exertion, irregular appetite, and emacia- 

 tion are all signs of tuberculosis, but 

 these may all be due to other affections, 



and it is not until an opportunity for Fig. 221. -Bacillus of Tuberculosis 



microscopic examination of expectorated 



matter or a post-mortem examination occurs that the existence of the 



tubercle is as a rule discovered. 



Morbid Appearances. — The structural changes which are met with 

 depend very much upon the organ which is affected. In the lungs there 

 are sometimes found miliary tubercles, while in other cases masses of 

 deposit an inch or more in diameter are met with. These larger masses 

 contain a quantity of soft yellow material in the centre. The bronchial 

 lymphatic glands, and also the glands in other parts of the body, are some-_ 

 times enormously enlarged, and present yellow softened masses in their 

 interior, and occasionally patches of calcareous or earthy deposit. 



Masses of tuberculous matter are occasionally found in the liver, spleen„ 

 and kidneys — more rarely in the intestines. 



On the lining membranes of the chest and belly isolated tuberculous 

 patches are met with, but the grapy form, as it is called, which is so 

 common in cattle, is not often seen in the horse. Cases of tuberculous 

 deposit in the bones have been recorded, and are probably more common 

 than is generally supposed, the fact being that cases of disease of the bones 

 in the horse are commonly classed under the head of caries or necrosis, and 



