524 THE MECHANICAL PROCESSES OF DIGESTION. [CH. xxxv. 



but a kind of double current is continually kept up among the 

 contents of the stomach, the circumferential parts of the mass 

 being gradually moved onward toward the pyloms by the con- 

 traction of the muscular fibres, while the central portions are 

 propelled in the opposite direction, namely towards the cardiac 

 orifice ; in this way is kept up a constant circulation of the con- 

 tents of the viscus, highly conducive to their thorough admixture 

 with the gastric fluid and to their ready digestion. 



Under ordinary circumstances, three or four hours may be 

 taken as the average time occupied by the digestion of a meal in 

 the stomach. But the digestibility and quantity of the meal, and 

 the state of body and mind of the individual, are important 

 causes of variation. The pylorus usually opens for the first time 

 about twenty minutes after digestion begins ; it, however, quickly 

 closes again. The intervals between its openings diminish, and 

 the periods during which it remains open increase, \intil towards 

 the end of the time it is permanently open, and the chyme can 

 pass freely into the duodenum. 



The results which have just been stated were made out previous to the 

 discovery of the Rcintgen rays. The subject has recently been taken up 

 again by Cannon. He gave an animal food mixed with bismuth subnitrate, 

 and obtained shadow photographs of the stomach, because the bismuth salt 

 renders its contents opaque. His results mainly confirm those of the earlier 

 investigators; the piincipal peristalsis occurs in the pyloric portion of the 

 stomach. The cardiac portion presses steadily on its contents, and as they 

 become chymified urge them onwards towards the pyloric portion : the latter 

 empties itself gradually through the pylorus into the duodenum, and in the 

 later stages, of digestion the cardiac part also is constricted into a tube. 



Influence of the Nervous System. The normal move- 

 ments of the stomach during gastric digestion do not appear to 

 be so closely connected with the plexuses of nerves and ganglia 

 contained in its walls as was formerly supposed. The action, 

 however, appears to be set up by the presence of food within it. 

 The stomach is, however, directly connected with the higher 

 nerve-centres by means of branches of the vagi and of the 

 splanchnic nerves through the solar plexus. 



The vagi (especially the left) contain the accelerator nerves of 

 the stomach ; when they are stimulated the result is peristaltic 

 movement. The sympathetic fibres are inhibitory ; when they 

 are stimulated peristalsis ceases. The cell stations on the course 

 of the vagus fibres are in the ganglion trunci vagi ; the post- 

 ganglionic fibres that issue from this ganglion are non-rnedullated. 



The sympathetic fibres leave the spinal cord by the anterior 

 roots of the spinal nerves from the fifth to the eighth thoracic. 

 They pass into the sympathetic system, have cell stations in the 



