4 THE RESPIRATORY SYSTEM. 



273 



modify the rate of respiration, and, secondly, that these impulses are of 

 two kinds, one tending to cause inspiration and the other expiration. 

 These fibres have their origin in the lungs, and their endings can be 

 stimulated by inflating, or sucking air out of, the lungs. Fig. 108 illustrates 

 the effect of suddenly distending the pulmonary alveoli with air ; the 

 diaphragm remains re- 

 laxed, inspiratory move- 

 ments cease, and the whole 

 chest is in the expiratory 

 position. This effect, 

 which is produced only 

 when the vagus nerves are 

 intact, and lasts only as 

 long as the distension per- FlG ' 107. -Stimulation of the central end of one 

 e vagus between X and X. The diaphragm 



sists, is known as vagus enters into continued contraction (inspiratory 



apnoea. Conversely, when position). 



air is sucked out of the lungs (negative ventilation) the diaphragm 

 is thrown into contraction, and the inspiratory movements are 

 increased. 



It is evident that distension of the pulmonary alveoli stimulates the 

 endings in the lungs of afferent fibres which pass up the vagus to the 



respiratory centre, and inhibits in- 

 spiration and brings about expira- 

 tory movements. On the contrary, 

 collapse of the pulmonary alveoli 

 stimulates the endings of afferent 

 fibres running in the vagi and con- 

 veying impulses to the respiratory 



FIG. 108. Apnrea produced by sudden centre whereby inspiratory move- 



-nts are evoked. 



The existence of such impulses 

 can be further demonstrated by connecting the vagus with a string 

 galvanometer; with each inspiration a deflection of the thread takes 

 place, which indicates the passage of an impulse along the nerve ; and, 

 under certain conditions, a similar deflection may also be observed 

 during expiration. In normal respiration these two sets of fibres are 

 alternately stimulated, each inspiration sending impulses along one set 

 of fibres and reflexly causing expiration, whereas each expiration gives 

 rise to impulses which bring about another inspiration. The electrical 

 variations in the vagus seem to show that the impulses leading to 

 expiration are the more important and more pronounced. 



Although these impulses help to maintain the normal rhythm of the 



18 



