326 MECHANISMS Of THE DIGESTIVE TRACT. 



excite vomiting, though, after paralysis of the abdominal muscles alone, 

 the expulsion of the gastric contents can be still brought about by the 

 agency of the diaphragm. The intrinsic contraction of the stomach 

 itself, though unable to empty this organ, as in vomiting, can cause the 

 escape of gas or of small quantities of food or of fluid, as occurs in 

 pyrosis, or water-brash. 



Since vomiting involves the co-ordinated activity of a number of 

 muscles, voluntary as well as involuntary, it is evident that the proper 

 carrying out of this process must depend on the integrity of the central 

 nervous system. Vomiting is generally said to occur through the 

 agency of a vomiting centre in the medulla oblongata. This has been 

 thought to be closely related in situation to the respiratory centre; 1 

 and it is probable that, like the latter, it is intimately related to 

 the central connections of the afferent and efferent fibres of the 

 vagus. 



Vomiting can be excited reflexly in many ways. The most usual 

 stimulus is one affecting the stomach. Both in this case, and in all 

 other cases when the exciting cause is some affection of the abdominal 

 viscera, the afferent impulses are probably carried by the vagi. 

 Vomiting can also be excited reflexly through the olfactory, the fifth 

 and glosso-pharyngeal, or the auditory nerve, or directly from the higher 

 parts of the brain. The efferent channels of the act are : The vagus, 

 to stomach, cardiac sphincter, oesophagus, larynx ; through chorda tym- 

 pani, and probably tympanic branch of glosso-pharyngeal to salivary 

 glands ; through phrenics and various spinal nerves to respiratory 

 muscles. 



Emetics, or substances which excite vomiting, may act either re- 

 flexly from the stomach (e.g. mustard and water), or directly on the 

 centre (apomorphin), or in both ways, as in the case of tartar emetic. 



MOVEMENTS OF THE INTESTINES. 



After the chyme has arrived in the duodenum, it has to be carried 

 slowly from one end of the intestine to the other, in order that the 

 digestive juices poured into different parts of the gut may have time to 

 complete their action on the food-stuffs, and that all assimilable con- 

 stituents may be absorbed. Thus only the indigestible portions of 

 food, together with a certain amount of epithelial cUbris and mucus, 

 arrive at the lower end, to be extruded in the act of defecation. 



The investigation of the mechanism of intestinal movements is beset 

 with many difficulties, so that we still find a considerable divergence 

 among various authors of repute, even as to the character of the move- 

 ments which are the subject of investigation. 



The following are the chief factors which contribute to this diver- 

 gence of results, and which must be taken into account in working on 

 the intestinal movements, namely, the varying reactions of the intes- 

 tines in different animals, the disturbing action of drugs and anaesthetics, 

 but, above all, the fact that each segment of the intestines is subject to 

 augmentor and inhibitory influences, partly originating in other portions 

 of the gut and transmitted along the walls of the intestine, partly 



1 Grimm, Arch. f. d. ges. Physiol., Bonn, 1871, Bd. iv. S. 205. The identity has been 

 contested by Harnack on pharmacological grounds ; Arch. f. exper. Path. u. PharmakoL, 

 Leipzig, 1874, Bd. ii. S. 154. 



