252 HAND-BOOK OF PHYSIOLOGY. 



Some persons possess the power of vomiting at will, without applying 

 any undue irritation to the stomach, but simply by a voluntary effort, 

 It seems also, that this power may be acquired by those who do not 

 naturally possess it, and by continual practice may become a habit. There 

 are cases also of rare occurrence in which persons habitually swallow their 

 food hastily, and nearly unmasticated, and then at their leisure regurgi- 

 tate it, piece by piece, into their mouth, remasticate, and again swallow 

 it, like members of the ruminant order of Mammalia. 



The various nerve-actions concerned in vomiting are governed by a 

 nerve-centre situate in the medulla oblongata. 



The sensory nerves are the fifth, glosso-pharyngeal and vagus princi- 

 pally; but, as well, vomiting may occur from stimulation of sensory 

 nerves from many organs, e.g. , kidney, testicle, etc. The centre may also 

 be stimulated by impressions from the cerebrum and cerebellum, so called 

 central vomiting occurring in disease of those parts. The eiferent im- 

 pulses are carried by the phrenics and the spinal nerves. 



Influence of the Nervous System on Gastric Digestion. The 

 normal movements of the stomach during gastric digestion are directly 

 connected with the plexus of nerves and ganglia contained in its walls, 

 the presence of food acting as a stimulus which is conveyed to the gan- 

 glia and reflected to the muscular fibres. The stomach is, however, also 

 directly connected with the higher nerve-centres by means of branches 

 of the vagus and solar plexus of the sympathetic. The vaso-motor fibres 

 of the latter are derived, probably, from the splanchnic nerves. 



The exact function of the vagi in connection with the movements of 

 the stomach is not certainly known. Irritation of the vagi produces con- 

 traction of the stomach, if digestion is proceeding; while, on the other 

 hand, peristaltic action is retarded or stopped, when these nerves are 

 divided. 



Bernard, watching the act of gastric digestion in dogs which had fis- 

 tulous openings into their stomachs, saw that on the instant of dividing 

 their vagic nerves, the process of digestion was stopped, and the mucous 

 membrane of the stomach, previously turgid with blood, became pale, 

 and ceased to secrete. These facts may be explained by the theory that 

 the vagi are the media by which, during digestion, an inhibitory impulse 

 is conducted to the vaso-motor centre in the medulla; such impulse being 

 reflected along the splanchnic nerves to the blood-vessels of the stomach, 

 and causing their dilatation (Rutherford). From other experiments it may 

 be gathered, that although division of both vagi always temporarily sus- 

 pends the secretion of gastric fluid, and so arrests the process of digestion, 

 being occasionally followed by death from inanition; yet the digestive 

 powers of the stomach may be completely restored after the operation, 

 and the formation of chyme and the nutrition of the animal may be 

 carried on almost as perfectly as in health. This would indicate the 



