ASTHMA. 439 



the instant of application of an irritant to the nerve ; " the lung 

 being, as I take it, seized and fixed by spasm, and immobility 

 of the bronchial muscles." — (Dr. J. C. Thoeowgood.) 



In the horse the arrest is always during the expiratory act, 

 which, in consequence, becomes prolonged, difficult, and calls 

 for a double contraction of the abdominal muscles to force the 

 air out of the distended and now immobile bronchial tubes. I 

 have noticed that very round-chested horses sometimes become 

 broken-winded without apparent cause, and I conclude that 

 difficult expiration may occur in them from limited thoracic con- 

 traction. When the conformation of the chest is round, respira- 

 tory movements, at least thoracic movements, are very limited 

 in extent ; for when the chest is naturally round, its conformation 

 is but little altered during the respiratory movements. An animal 

 so formed is incapable of taking a deep inspiration, and as 

 incapable of performing a strong expiration ; the lungs of such 

 contain much residual air, the thoracic walls are more resonant 

 on percussion, and in them probably arrest of expiration is most 

 easily induced. Every horseman knows that round-chested 

 horses, although apt to put on fat and look well, are not so 

 capable of performing the same severe and fast work as deep- 

 chested animals ; in fact that they are " not so good in their wind." 



The abnormal condition of the bronchial tubes and air vesicles 

 is at first due to purely nervous derangement, but this is 

 very rapidly succeeded by organic or structural change. Infla- 

 tion of the air cells to excess prevents the free circulation of 

 the capillary blood-vessels, and thus the nutrition of the bron- 

 chial muscle and mucous membrane is weakened ; degeneration 

 succeeds, and the lungs become more and more emphysematous ; 

 the air cells undergo dilatation, their parietes become stretched 

 and disorganized, and air now infiltrates into the meshes of the 

 connective tissue of the lungs. In this manner both vesicular 

 and interlobular emphysema may be induced. Various other 

 changes may occur in the thoracic organs, the most constant 

 being hypertrophy and dilatation of the right side of the heart. 

 By bearing these facts in view, much of what seems irrecon- 

 cilable may be understood, and what have been looked upon as 

 causes will be seen to be but mere effects. 



Tlie morbid anatomy of broken wind differs according to its 

 duration at the time of the animal's death, which is generally due 



