136 THE ENGINES OF THE HUMAN BODY 



the nose becomes obstructed we can breathe through the 

 mouth, but then our mouths become dry and the air is 

 not properly filtered or warmed. Passing down the 

 throat the inrushing tide of air reaches, just behind and 

 below the root of the tongue, the most delicately con- 

 trived doorway in all the body — a doorway leading into 

 the larynx, trachea or windpipe, and lung passages or 

 bronchi. When we swallow, the respiratory gateway 

 closes the instant that anything solid or liquid enters 

 the throat or pharynx (fig. 37). But when the tide of 

 air enters it opens all the wider to let it rush towards 

 the lungs. 



There is a second gateway immediately under the first, 

 known as the glottis, situated within the larynx, a structure 

 which we can readily feel in our necks at the upper end 

 of the windpipe. The glottis, through which the breath 

 has to pass, is a triangular doorway with a sliding curtain 

 at each side that can swing inwards until only a chink is left 

 between them, or can be closed tightly so that the glottis 

 or doorway is quite shut. These two swinging curtains 

 are the vocal cords — the warning horn of the human 

 machine (fig. 37). Nature, following her usual habits of 

 economy, has set it right in the breathing highway, so that 

 she may use the respiratory bellows to sound a warning, 

 utter a signal of distress, or give out those brain-signals 

 which we call speech. The curtains of the glottal door- 

 way are manipulated by a battery of very small muscular 

 engines. As the inrushing breath approaches they pull 

 the two curtains widely apart ; but if it should happen that 

 the air contains an irritating vapour, then they keep them 

 tightly shut, and we have the feeling of being suffocated. 



Having passed the gateway formed by the vocal cords, 

 the ingoing tide of air descends in the neck through the 

 windpipe, which, when it has passed some way within the 

 thoracic cage, divides into two branches or bronchi, one 

 for each lung. We shall follow the current which enters 

 the right lung. If you would obtain a true conception 

 of the passages along which the current of air rushes 

 until the final respiratory chambers of the right lung are 



