MECHANICS OF RESPIRATORY MOVEMENTS 1049 



the bronchial muscle. During inspiration this expanding force is in- 

 creased, so that in the presence of bronchial constriction the access of air 



FIG. 490. Tracings of the volume changes of the lung, with constant variations of 

 trachea] pressure. (BKODIE and DIXON.) T. P. tracheal pressure. L.V. lung 

 volume. B.P. blood-presure (Zero B.P. 17mm. below time marker). Showing 

 constriction of bronchial musculature as a result of vagus excitation. 



FIG 491. Tracing showing inhibitory effect of vagus on the bronchial tonus pro- 

 duced by O'Ol grm. pilocarpine. 



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, 



