254 APPLIED PHYSIOLOGY 



unconsciously by patients who seek to stop vomiting by 

 the taking of a series of deep breaths. 



The vomiting centre can be excited either directly or 

 reflexly. The former mode of excitation occurs in the 

 vomiting of cerebral anaemia or hyperaemia, in intoxica- 

 tions e.g., uraemia in cases of intracranial disease, 

 and in the vomiting of psychical origin. Reflex irrita- 

 tion of the centre can be brought about by stimuli 

 reaching it from many peripheral sources. 



Duodenal Digestion. 



In the duodenum the chyme discharged from the 

 stomach meets the bile and pancreatic juice. The mix- 

 ture of the two latter forms a yellowish-green fluid of a 

 specific gravity of 1010, and of a neutral or slightly 

 alkaline reaction.* The addition of the acid chyme to 

 this throws down a precipitate which consists of mucin, 

 bile acids, and bile pigments, but which does not contain 

 pepsin ; and if there be sufficient chyme to give it a 

 decidedly acid reaction, digestion goes on just as in the 

 stomach, and this undoubtedly must happen to a con- 

 siderable extent when the stomach produces an excess 

 of acid. As a rule, however, beyond the first few inches 

 of the duodenum, there is no free mineral acid present, 

 and it may be for this reason that duodenal ulcers are 

 only met with for a short distance beyond the pylorus. 



Pure human pancreatic juice, as derived from a 

 fistula,t is a clear fluid of specific gravity 1007, alkaline 



* Boas, Zeit.f. Klin. Med., 1890, xvii. 155. 



t Glaessner, Zeit. f. Physiol Chemie, 1903-4, xl. 465. 



