1050 PHYSIOLOGY 



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 walls is changed into a pressure tending to obliterate their 

 lumen and so impede the out flow 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 

 contractions 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 inspiratory muscles ; the expiratory muscles, such 

 as the abdominal, will be found to be quite flaccid even during expiration. 



