222 A MANUAL OF PHYSIOLOGY 



on the spinal bulb may produce it. But it is also seen, more 

 or less perfectly, in normal sleep, especially in children, and 

 in morphia and chloral poisoning. A periodic change in the 

 activity of the respiratory centre, corresponding to the 

 change in the vaso-motor centre which is credited with the 

 production of Traube-Hering oscillations in the blood- 

 pressure (p. 250), has been suggested as the cause, but there 

 is no certainty as to this. 



In frogs, Cheyne-Stokes' breathing has been observed as 

 the result of interference with the circulation in the spinal 

 bulb, * drowning,' or ligature of the aorta, and also as a con- 

 sequence of removal of the brain, or parts of it (hemispheres 

 and optic thalami) (Langendorff, Sherrington, etc.). 



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, some- 

 times 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 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 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 



