458 



ORGAN SYSTEMS OF MAN 



ally expands until full; the reverse process 

 takes place as digestion is completed and 

 the food is moved along to the small in- 

 testine. Fluids such as water pass through 

 the stomach in a few minutes, whereas more 

 solid foods remain from 3 to 5 hours, de- 

 pending on their nature. Some foods digest 

 more slowly than others, for example, those 

 rich in fats. The food is retained in the stom- 

 ach and churned until it resembles a thick 

 soup, which is called the chyme. Peristaltic 

 waves begin in the cardiac region and move 

 toward the pyloric end, so that the food is 

 constantly being forced into that end. So 

 long as the pyloric valve remains closed, the 

 food cannot pass into the intestine, but 

 must continue to be mixed back and forth 

 until the proper consistency is reached. The 

 pyloric valve then opens intermittently, al- 

 lowing small amounts of the chyme to pass 

 in spurts into the upper end of the small 

 intestine. 



These movements are under the influ- 

 ence of the autonomic nervous system. 

 Parasympathetic fibers reach the stomach 

 through the vagus nerves and their action 

 increases the intensity of the peristaltic 

 waves and augments the flow of gastric 

 juice. The opposite action is brought about 

 by impulses coming through fibers from 

 the sympathetic system. Undue emotional 

 strain during a meal excites the sympathetic 

 fibers excessively, thereby slowing up stom- 

 ach activity, which is not conducive to good 

 health. For this reason, meal time is no 

 time to discuss weighty or controversial 

 problems. 



Provision has been made in most animals 

 for removing anything taken into the stom- 

 ach that does not "set well." This is par- 

 ticularly true in carnivores, who are apt to 

 eat slightly decayed food that might be 

 toxic. Regurgitation, as this chain of events 

 is called, is just the reverse of swallowing, 

 culminating in a complete evacuation of 

 the stomach. Just prior to the major event 

 a deep breath is taken in order to hold the 

 diaphragm down; a convulsive contraction 



of the abdominal muscles then follows 

 which presses the stomach up against the 

 rigid diaphragm. The sphincter leading into 

 the esophagus relaxes and the stomach con- 

 tents are expelled almost explosively. The 

 action also resembles swallowing in that 

 once it starts it continues until the job is 

 done, regardless of the will of the owner. 

 This safety mechanism is a feature of im- 

 portance in the survival of a species. 



Digestion in the stomach 



The millions of tiny glands in the walls of 

 the stomach secrete gastric juice, a highly 

 acidic, watery fluid that contains the pro- 

 tein-splitting enzyme, pepsin. Between 400 

 and 800 cc. of gastric juice are produced 

 during the digestion of an average meal. 

 The flow of gastric juice, like the flow of 

 saliva, is under the influence of the nervous 

 system, at least in part. Everyone has ex- 

 perienced "mouth watering" when hungry 

 and within sight or smell of good food; the 

 stomach "waters" the same way and prob- 

 ably at the same time. Just how this func- 

 tions was an attractive subject for investiga- 

 tion even for early biologists, and in recent 

 years research has been so fruitful that the 

 entire mechanism is rather well understood. 

 Let us follow briefly the history behind 

 these discoveries. 



At the time of the signing of the Declara- 

 tion of Independence in this country, a 

 clergyman in Italy attempted some experi- 

 ments on the activities of his own stomach. 

 This amazing Spallanzani swallowed small 

 metal cages containing bits of meat and 

 after these were left in his stomach for 

 varying lengths of time they were retrieved 

 by means of an attached string. He noticed 

 that the tiny bits of meat had disappeared 

 and concluded that they must have gone 

 into solution and therefore were digested. 

 Another notable series of experiments, be- 

 gun about 40 years later and continued for 

 many years, was performed by an Ameri- 

 can army surgeon, Dr. William Beaumont, 

 who had the good fortime of treating a man 



