DIGESTION 253 



place even when all the abdominal muscles are paralyzed. 



I That the pylorus does not remain tightly closed during 



\ the whole process is indicated by the clinical facts 



jthat bile, gall-stones, and intestinal worms may all be 



\vomited. 



The simultaneous contraction of the diaphragm and 

 the abdominal muscles, which is peculiar to vomiting, 

 raises enormously the intra-abdominal pressure, and 

 forces blood up into the heart. The blood-pressure is 

 thereby increased, and the feeling of faintness which 

 precedes the act of vomiting is relieved. This explains 

 the great relief which immediately follows vomiting in 

 cases of sea-sickness; and, indeed, the tendency to vomit- 

 ing which is exhibited in all cases of cerebral anaemia 

 must really be regarded as a conservative measure 

 which is calculated to increase cerebral blood-pressure. 



The complicated mechanism by which vomiting is 

 brought about is controlled by a special centre situated 

 in the medulla in the neighbourhood of the calamus 

 scriptorius, and close to the respiratory and vasomotor 

 centres. Destruction of this centre renders vomiting 

 impossible, whilst the application to it of a dilute solu- 

 tion of apomorphine excites the act in an extreme 

 degree. 



The proximity of the respiratory and vomiting centres 

 explains how it is that irritation of the former, as in 

 dyspnoea, often induces nausea or even actual vomiting 

 as well. On the other hand, it has been found that the 

 induction of apnoea i.e., the temporary inhibition of the 

 irritability of the respiratory centre can arrest the act 

 of vomiting for a time, a fact which is often acted upon 



