288 RESPIRA TION 



breathing becomes quite deep or actually dyspnoeic. The store of 

 oxygen is replenished by this thorough ventilation of the lungs, the 

 changes 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 apnoea 

 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 adminis- 

 tration 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 dia- 

 phragm, which causes a sudden inspiration. The abrupt closure of 

 "the glottis cuts this short and gives rise to the characteristic 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 originating 

 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 con- 

 tractions of the diaphragm can sometimes be elicited by stimulation 



