1048 PHYSIOLOGY 



therefore at the moment at which the manometer is put into connection 

 with the tracheal tube, and a rise of the mercury in the manometer is there- 

 fore produced. By this method was obtained the tracing shown in Fig. 489. 

 By the second method artificial respiration at a constant pressure is made 

 use of. Any changes in the bronchioles will in this case affect the volume 

 of air entering the lungs at each stroke of the pump, and can be measured 

 by recording either the passive respiratory movements of the chest or the 

 changes in volume of a lobe of the lung enclosed in a plethysmograph 





FIG. 489. Tracings of blood -pressure (middle curve) and of intratracheal pressure 

 (upper curve) taken by Einthoven's differential manometer. Between Q and Q' 

 the peripheral end of one vagus was stimulated. Time marking = seconds. 



(Brodie and Dixon). By both these methods it has been shown that 

 stimulation of the peripheral end of either vagus causes constriction of the 

 bronchioles (vide Figs. 490 and 491). As a rule there is little tonic action of the 

 vagi, section of both vagi leaving the respiratory pressure curve unaltered 

 or lowering it slightly by 2 to 10 mm. H 2 0. It is very easy to bring about 

 a vagus tonus by allowing the animal to inhale air containing 3 to 4 per 

 cent, carbon dioxide. A peripheral tonus may also be produced by ad- 

 ministration of muscarine or pilocarpine. In the latter case Brodie and 

 Dixon have shown that stimulation of the vagus may cause relaxation 

 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 



