EFFECTS OF BRONCHIAL CONSTRICTION. 



employed (Fig. 180); the effect is increased within limits, by increasing the 

 duration of the stimulus. The latent period which elapses between the 

 beginning of the excitation and the commencement of the bronchomotor 

 effect, lies between I'O and 1'5 seconds. 



It might be thought that this bronchial spasm was directly caused by 

 the fall of blood pressure, due to the action of the vagus on the heart. It is 

 not possible to exclude this explanation by administration of atropin, since 

 this drug paralyses completely the bronchomotor fibres of the vagus. Eintho- 

 ven has shown, however, that there is no proportionality between the effect on 

 the heart and the amount of bron- 

 chial constriction, and that, in fact, 

 a well-marked and typical bronchial 

 constriction may be brought about 

 by vagal stimulation, even after the 

 death of the animal and conse- 

 quent cessation of the blood flow. 



Tonic action of the vagus. 

 In the dog there is, as a rule, 

 very slight evidence of a tonic 

 influence of the vagus on the bron- 

 chial muscle. Section of both vagi 

 either does not alter the respiratory 

 curve (Einthoven's method), or 

 lowers it by 2 to 10 mm. water. 

 It is, however, in all cases, easy to 

 bring about a vagus tonus by allow- 

 ing the animal to inhale air con- 

 taining an excess (3 or 4 per cent.) 



of C0 2 . As the 



C0 2 tension in 



FIG. 180. Diminution of respiratory excursions 

 produced by spasm of bronchial muscle (arti- 

 ficial respiration at constant pressure). A' = 

 thoracic wall, A 2 = diaphragm. Vagus 

 stimulated between P and P'. Einthoven. 



the blood rises, the bronchi constrict 

 slowly. This constriction is at once 

 cut short by dividing both vagi. 

 It is not due to commencing 

 asphyxia (oxygen hunger), since it 

 is not produced by allowing the 

 animal to inhale pure nitrogen. 



Chauveau 1 states that in the 

 horse the vesicular murmur dis- 

 appears after section of the vagi. 

 This fact would tend to point to 

 a marked tonic action of the vagi 

 on the bronchial muscles in this 

 animal. 



When present the vagus tonus may present slight slow rhythmical oscilla- 

 tions, and may sometimes be inhibited by stimulation of the central end of a 

 sensory nerve such as the sciatic. In one case, Einthoven observed a reflex 

 broncho-constriction on stimulating the central end of the sciatic in a curarised 

 dog. There seems to be no satisfactory evidence for assuming the existence 

 of broncho-dilator fibres in the vagus. 



The effects of bronchial constriction ; Asthma. Under the in- 

 fluence of vagal stimulation or of CO.,, the pressure necessary 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 

 1 Quoted by Einthoven, loc. cit., S. 373. 



