DIGESTION 179 



gastric juice with the food, but also serve to eject the liquefied portions of 

 the food into the small intestine. 



The movements of the human stomach as described by Beaumont, as 

 well as the movements of the dog's stomach as stated by different observers 

 are not in agreement in all respects, and are, moreover, open to question 

 for the reason that they were not observed under strictly physiologic con- 

 ditions. The more recent investigations of Cannon have thrown new light 

 on this subject. By means of the Rontgen rays he has been enabled to 

 study the movements in the living animal and under normal conditions. 

 The animal (the cat) was fed with bread and milk, to which was added 

 subnitrate of bismuth. This substance, being opaque, rendered the move- 

 ments of the stomach walls visible on the fluorescent screen. With paper 

 placed over the screen it was possible to sketch the change in shape that 

 the stomach undergoes at different periods of the digestive act. The re- 

 sults of these investigations will be referred to in following paragraphs. 



As a result of many methods of investigation it has become apparent that 

 the activities of different portions of the stomach, whereby food is admitted 

 into it, retained there, triturated and mixed with the gastric juice and finally 

 discharged into the duodenum, are due (i) to causes resident in the stomach 

 walls and the stomach contents and (2) to nerve impulses descending from 

 the central nerve system through the vagi and splanchnic nerves. 



At the end of a digestive period the walls of the stomach contract and 

 almost obliterate its cavity. The sphincter cardiae and sphincter pylori 

 are also contracted and the orifices they surround are more or less tightly 

 closed. 



The Movements of the Sphincter Cardia. The sphincter cardice muscle 

 surrounding the esophago-gastric orifice is always, under normal conditions, 

 tonically contracted and the orifice closed. This contraction is due, first, 

 to causes resident in the muscle itself, and second, to the arrival of nerve 

 impulses descending the vagus nerves from the medulla oblongata. That 

 the contraction can be brought about by inherent causes is shown by the 

 fact that it persists for from 24 hours to several days after division of all 

 nerves distributed to it. The contraction may be so pronounced as*to 

 offer considerable resistance not only to the passage of food but even to the 

 introduction of a sound into the stomach. (Cannon.) That the normal 

 contraction is under the influence of the central nerve system is shown by 

 the effects which follow division and stimulation of the peripheral end of 

 the vagus. Thus, if it is stimulated with weak induced currents, the con- 

 traction of the sphincter is somewhat inhibited and the orifice enlarged; 

 if it is stimulated with strong currents the contraction is markedly increased 

 and the orifice diminished. Apparently there are in the vagus two sets of 

 efferent nerve-fibers, one of which inhibits while the other augments the 

 contraction, and corresponding to the nerves there must be In the medulla 

 oblongata two centers from which they arise, an augmentor and an 

 inhibitor. 



Observation has also shown that in the act of deglutition there is an 

 inhibition of the contraction of the sphincter muscle as the bolus of food 

 reaches the lower end of the esophagus, under the progressive movement 

 of the peristaltic contraction. With the passage of food into the stomach 

 the tonic contraction again supervenes. If the acts follow each other in 



