g6 IIORSE-BACK RIDING. 



secretion. This view is the older one — the one advo- 

 cated by Galen and Celsius ; the other, that it is 

 caused by bronchial spasm, is the theory of Van 

 Helmont and Willis. With Beau, I believe that 

 nervous asthma is only an intermittent bronchial 

 catarrh, and that the dyspnoea, the feature of this 

 disease, is caused by the resistance which the mucus, 

 in the small bronchial tubes, offers to the passage of 

 air. It varies in intensity with the degree in which 

 the bronchi are obstructed. 



Sonorous and sibilant rales are produced by the air 

 passing through these parts of the bronchial tubes, 

 which have a smaller calibre, on account of the 

 mucous deposit. They are louder and more numer- 

 ous during expiration, as then we have not only the 

 obstruction in the air tubes, but a diminution in the 

 size of the tubes themselves, due to the contraction of 

 the lung. Sometimes the obstruction is complete, 

 and then, no air passing, there is absence of all sound 

 in that portion of the lung — this is known as absence 

 of vesicular murmur. Rales and absence of vesicular 

 murmur may alternate with each other, since cough- 

 ing may render a complete obstruction incomplete, 

 or vice versa. Air is sometimes entrapped between 

 the terminal extremity of the bronchi and the mucous 

 obstruction. 



The movement of expiration or of coughing tends 

 to compress it, but this tendency is resisted by its 

 elasticity, and the air cells are dilated, constituting 



