ASTHMA AND BROKEN WIND. 167 



especially of the vesicular form, has been found in many- 

 horses which have not at any time shown symptoms of 

 broken wind. 



Now, therefore, although it is probable that the amount of 

 respiratory derangement bears a close relation to the ex- 

 tent of the emphysematous condition of the lungs, we can 

 nevertheless only regard these lesions as the occasional re- 

 sult of some other diseased condition, and in no measure as 

 the prime cause. 



Eegarding broken wind as dependent on disturbed inner- 

 vation, traceable to derangement of the digestive system, 

 the phenomena of this disease, and the variable pathological 

 lesions found, may be readily accounted for. 



Between the sub-epithelial mucosa and the submucosa of 

 the small bronchial tubes there is a continuous layer of 

 circular non-striped muscular tissue. This layer is especially 

 conspicuous and important in the smaller microscopic 

 bronchi (Klein), and by contracting aids the expulsion of 

 tidal air from the lungs in expiration. 



This muscle in broken wind is first spasmodically con- 

 tracted, and then afterwards becomes paralyzed, and under- 

 goes degenerative changes. This is owing to the irritation 

 set up by indigestible food acting on the gastric branches of 

 the pneumogastrics, and reflected through the pulmonary 

 branches of the same nerves. This spasmodic contraction 

 or paralysis arrests both the respiratory acts, especially the 

 expiration, which becomes double, and requires a double 

 contraction of the abdominal muscles to force out the 

 air from the distended air-cells and now inactive bronchial 

 tubes. 



Kound-chested horses sometimes become broken-winded 

 without any apparent cause, and difficulty in expiration may 

 be attributed in such cases to the limitation of the thoracic 

 contraction. It will easily be seen that when the shape of 



