IV THE NERVES OF RESPIRATION 159 



respectively, reach the diaphragm and the intercostal 

 muscles. These immediately contract, and thus an 

 inspiration takes place. Thereupon the impulses cease, 

 and are replaced by other impulses, which though starting 

 from the same centre pass, not to the diaphragm and 

 external intercostal muscles, but to other, expiratory, 

 muscles, which they throw into contraction, and thus 

 expiration is brought out. As a general rule the inspira- 

 tory impulses are much stronger than the expiratory ; 

 indeed, in ordinary quiet breathing expiration is chiefly 

 brought about, as we have seen, by the elastic recoil of 

 the lungs and chest walls ; these need no nervous imjjulses 

 to set them at work, as soon as the inspiratory impulses 

 cease and tlie diaphragm and other inspiratory muscles 

 leave off contracting, thej' come of themselves into action. 

 But, in laboured breathing, very powerful expiratory 

 impulses may leave the respiratory centre and pass to the 

 various muscles whose contractions help to drive the air 

 out of the chest. 



Every day experience shows that no function of the body 

 is more obviously subject to sudden and marked changes 

 than is the respiration. It is quickened by exercise, 

 quickened or slowed by emotions ; hurried by stimulation 

 of the skin, as by a dash of cold water, or brought to a 

 standstill by stimulating the mucous membrane of the 

 nose by a pungent vapour such as strong ammonia. The 

 changes involved in si^ezing, laughing, coughing, <S:c., are 

 profound and peculiar. Finally we can control our 

 respiration by an effort of the will within very wide limits 

 and in almost any desired way. The mechanism 

 involved in the production of all these changes is corre- 

 spondingly complicated ; but certain broad facts are fairly 

 simple, and to these we may now turn. 



The main trunk of the vagllS nerve (see Lesson XI.) 

 gives off a branch to the larynx as it passes down the neck 

 (Fig. 49, S.Lr.). If the vagus be cut hdoio the point oj 

 exit of this nerve (as at x, Fig. 49) and the upper (central) 



