1098 PHYSIOLOGY 



of the bronchioles, so that this nerve appears to contain both motor and 

 inhibitory fibres to the bronchioles. 



THE EFFECTS OF BRONCHIAL CONSTRICTION : ASTHMA. Under 

 the influence of vagal stimulation or of carbon dioxide, the pressure neces- 

 sary to drive the normal amount of air into the lungs may be raised in the 

 dog from 125 to 300 mm. H 2 0. We should therefore expect that, in cases 

 where bronchial constriction is present, there would be difficulty both in 

 inspiration and expiration. There is however a difference in the mechanical 

 conditions of the bronchi during the two phases of a respiratory move- 

 ment. Normally the elastic structure of the lungs is drawing upon the 

 bronchial wall, tending to maintain it patent, and so opposing the action of 



Fio. 503. Tracing showing inhibitory effect of vagus on the bronchial tonus pro- 

 duced by 0*01 grra. pilocarpine. 



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

 creased, so that in the presence of bronchial constriction 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 walls 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 1un<js and becomes slow 



