RESPIRATION 167 



sidered as the organ of voice. In connection with respiration, 

 it must be regarded as primarily a valve which closes the 

 entrance to the windpipe during swallowing. It is overhung 

 by a leaf-like appendage the epiglottis formed of exceed- 

 ingly elastic tissue. It was thought until lately that the 

 epiglottis drops over the aperture of the larynx when food is 

 passing down the gullet, and springs up again as soon as the 

 act of deglutition is over ; but recent observations have 

 shown that during deglutition the epiglottis is pressed against 

 the back of the tongue, and that the closure of the larynx is 

 effected by its own sphincter muscles. The mucous membrane 

 of the larynx is extremely sensitive to stimulation by anything 

 which would be prejudicial to the tissue of the lungs. When 

 its sensory nerve the superior laryngeal is stimulated, the 

 larynx closes. It is the agent in carrying out many reflex 

 actions, in which not the larynx only, but also the muscles of 

 the chest and diaphragm, take part. For example, it imme- 

 diately stops inspiration if an irritating vapour is present in 

 the air. It stops respiration if any foreign body, such as a 

 crumb of bread or a drop of water, touches the mucous 

 membrane. When the trunk of the nerve is stimulated by an 

 electric current, respiration is inhibited. Further, under suit- 

 able stimulation the nerve brings about respiratory move- 

 ments in which inspiration is gentle and expiration sudden, 

 violent, convulsive. Rib-muscles and diaphragm combine to 

 produce a cough, which ejects the noxious body. Again, its 

 stimulation in a different way probably helps to produce con- 

 striction of the smaller bronchi which regulate the amount of 

 air supplied to the air-cells of the lungs ; although this con- 

 striction may be largely due to a reflex which starts in the air- 

 cells. The epithelium of the air-cells has an immensely rich 

 supply of sensory nerves. In some persons this protective 

 mechanism is very prone to overact its part. A little dust or 

 foul gas in the air leads to such marked contraction of the 

 bronchi that respiration becomes very difficult. Such an 

 exaggerated tendency to reflex action constitutes the neurosis, 

 asthma. In this malady the mechanism is unduly sensitive. 

 Very slight stimulation leads to a maximum discharge of 

 impulses to the muscular tissue of the bronchi. 



The trachea has a length of about 4 inches. It extends from 



