3 i2 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



is, however, a difference in the mechanical conditions of the bronchi during 

 the two phases of a respiratory movement. Normally the elastic structure of 

 the lungs is drawing upon the bronchial wall, tending to maintain it patent, 

 and so opposing the action of the bronchial muscle. During inspiration 

 this expanding force is increased, so that in the presence of bronchial con- 

 striction the access of air is rendered the easier, the more powerful the 

 contraction of the inspiratory muscles. In expiration all parts of the lung 

 collapse, drawing with them the chest wall ; the pull of the lung tissue on the 

 bronchial wall is lessened, but is still present. If, however, the expiratory 

 muscles contract vigorously, the intrapleural pressure becomes positive, and 

 the pull of the lung tissue on the bronchial Avails is changed into a pressure 

 tending to obliterate their lumen, and so impede the outflow of air. 



It is evident, therefore, that in the presence of a spasmodic contraction of 

 the bronchial muscles the inspiration will be forcible and rapid, but all con- 

 tractions of muscles must be avoided, so far as possible, during expiration, 

 which must be left to the elastic reaction of the lungs, and becomes slow and 

 prolonged. Moreover, it will be of advantage to keep the lung as nearly as 

 possible in the inspiratory position, so as to reinforce the elastic forces which 

 dilate the bronchioles and aid expiration. We thus get the typical breathing 

 which occurs in man in cases of spasm of the bronchial muscles, known as 

 asthma nervosum. This type of breathing is often described as being marked 

 by expiratory dyspnoea. This description is, however, erroneous. The 

 muscles which in these cases are contracted to their uttermost, are the inspira- 

 tory muscles ; the expiratory muscles, such as the abdominal, will be found to 

 be quite flaccid even during expiration. 



