336 DIGESTION 



will induce vomiting, the stomach alone contracts during the act. 

 But in the adult such a contraction is ineffectual, and the same la 

 the case in animals, for a dog under the influence of a moderate 

 dose of curara, which paralyzes the voluntary muscles but not the 

 stomach, cannot vomit. 



The nerve-centre is in the medulla oblongata. It may be 

 excited by many afferent channels : the sensory nerves of the fauces 

 or pharynx, of the stomach or intestines (as in strangulated hernia) , 

 of the liver or kidney (as in cases of gall-stone or renal calculi), of 

 the uterus or ovary, and of the brain (as in cerebral tumour), are 

 all capable, when irritated, of causing vomiting by impulses passing 

 along them to the vomiting centre. 



The vagus nerve in man certainly contains afferent fibres by the 

 stimulation of which this centre can be excited, for it has been 

 noticed that when the vagus was exposed in the neck in the course 

 of an operation, the patient vomited whenever the nerve was 

 touched (Boinet, quoted by Gowers). In meningitis, vomiting is 

 often a prominent symptom, and is sometimes due to irritation of 

 the vagus nerve by the inflammatory process. 



Some drugs act as emetics by irritating surfaces in which efficient 

 afferent impulses may be set up, the gastric mucous membrane, 

 for example; sulphate of zinc and sulphate of copper act mainly 

 in this way. Apomorphine, on the other hand, stimulates the 

 centre directly, and this is also the mode in which vomiting is pro- 

 duced in certain diseases of the medulla oblongata. The efferent 

 nerves for the diaphragm are the phrenics, for the abdominal 

 muscles the intercostals. The impulses which cause contraction 

 of the stomach pass along the vagi. Dilatation of the cardiac 

 orifice is brought about by the inhibitory fibres in the vagus already 

 mentioned. 



SECTION III. THE CHEMISTRY OF THE T IGESTIVE JUICES. 



Ferments. The chemical changes wrought in the food as it 

 passes along the alimentary canal are due to the secretions of 

 various glands which line its cavities or pour their juices into it 

 through special ducts. These secretions owe their power for the 

 most part to substances present in them in very small amount, 

 but which, nevertheless, act with extraordinary energy upon the 

 various constituents of the food, causing profound changes with- 

 out, upon the whole, being themselves used up, or their digestive 

 power affected. The active agents are the enzymes, sometimes 

 spoken of as unformed or unorganized ferments unorganized 

 because their action does not depend upon the growth of living 

 cells, which was long supposed to be the case for some other fer- 

 ments, such as yeast. Since it has been shown that specific enzymes 

 can be separated from cells which were formerly believed to act 



