RESPIRATION 239 



in the excitability of the respiratory centre due to lack of oxygen 

 disappear (perhaps by oxidation of the lactic acid), and the 

 centre relapses into a period of repose. During this period 

 of apncea the oxygen pressure sinks once more to the point 

 at which the change in the excitability of the respiratory centre 

 by carbon dioxide occurs, and the breathing again starts. In 

 pathological cases the want of oxygen may be associated either 

 with deficient circulation through the bulb-centre or with deficient 

 intake by the lungs. The administration of oxygen through a 

 mask has been shown in such cases to abolish the periodicity in 

 the respiration, and to render it more normal. 



Peculiarly modified, but more or less normal, respiratory acts 

 are coughing, sneezing, yawning, sighing, and hiccup. 



A cough is an abrupt expiration with open mouth, which forces 

 open the previously closed glottis. It may be excited reflexly 

 from the mucous membrane of the respiratory tract or stomach 

 through the afferent fibres of the vagus, from the back of the 

 tongue or mouth, and (by cold) from the skin. 



Sneezing is a violent expiration in which the air is chiefly 

 expelled through the nose. It is usually excited reflexly from 

 the nasal mucous membrane through the branch of the fifth 

 nerve which supplies it. Pressure on the course of the nasal 

 nerve will often stop a sneeze. A bright light sometimes causes 

 a sneeze, and so in some individuals does pressure on the supra- 

 orbital nerve, when the skin over it is slightly inflamed. 



Yawning is a prolonged and very deep inspiration, sometimes 

 accompanied with stretching of the arms and the whole body. 

 It is a sign of mental or physical weariness. 



A sigh is a long-drawn inspiration, followed by a deep expiration. 



Hiccup, or hiccough, is due to a spasmodic contraction of the 

 diaphragm, which causes a sudden inspiration. The abrupt 

 closure of the glottis cuts this short and gives rise to the character- 

 istic sound. The following readings of the intervals between 

 successive spasms were obtained in one attack : 13 sees., 12 sees., 

 15 sees., 9 sees., 14 sees., etc. i.e., one-fourth or one-fifth of the 

 frequency of the ordinary respiratory movements. The mere 

 fixing of the attention on the observations soon stopped the hiccup. 



Hiccup is generally considered to be a reflex movement, brought 

 about through the respiratory centre by afferent impulses origi- 

 nating in the stomach. The irritation may be merely due to 

 some slight digestive disturbance set up by overfilling of the 

 stomach, perhaps. This is exceedingly common in infants. But 

 persistent hiccup may also be a distressing symptom of very 

 formidable diseases for example, carcinoma of the pylorus. 

 Experimentally, reflex contractions of the diaphragm can some- 

 times be elicited by stimulation of the central end of the vagus at 



