ISS TMF INTFRNAL nNVIRONMFNT OF- Till BODY Part III 



sistency. Fluids (alcohol is absorbed directly into the blood) leave almost 

 immediately, and solids last; carbohydrates, proteins, and fats leave the 

 stomach in that order. An ordinary mixed meal remains in the human stomach 

 from three to tour hours. 



Observations and Experiments. The famous experiment which first 

 showed that food undergoes chemical changes during digestion was performed 

 by a pioneer in experimental zoology. Rene Reaumur ( I6(S3-1757). It proves 

 first of all that an inquiring mind can discover something with simple equip- 

 ment. Reaumur placed bits of meat in small, perforated metal tubes fastened 

 to threads and fed them to his falcon and some other pet animals. When he 

 recovered them from the stomachs, he found the meat partially dissolved. 

 Soon after that he fed bits of sponge to a chicken and later pulled them 

 forth drenched with gastric juice. He next discovered that meat would be 

 dissolved if dropped into an open dish of gastric juice. Years later the Rus- 

 sian physiologist, Pavlov (1849-1936), discovered that the gastric glands 

 were stimulated not only when a dog took food into its mouth, but when it 

 smelled food, or heard a bell which it associated with food. 



Experiments with balloons have shown that hunger pangs are due to the 

 futile contractions of an empty stomach. The subject of such experiments 

 swallows a soft balloon which is then partially inflated and attached by a 

 tube to an apparatus which records the changes of pressure on the balloon. 

 Every time the stomach contracts it squeezes the balloon, causing a lift in 

 the writing point of the kymograph. At the same time the subject, who does 

 not see the record, presses a button because he feels a hunger pang (Fig. 

 11.16). As the experiment proceeds, the signal of the hunger pang and the 

 record of a squeeze on the balloon occur regularly at the same time. Con- 

 tractions during hunger do not seem to be different from ordinary ones 

 except that they are stronger. In moderate hunger the pangs are felt for a 

 time and then cease, like a recovery from frustration. Observations and photo- 

 graphs of the movements of the stomach are made by x-ray after a meal 

 containing some harmless substance, usually barium sulfate, that appears 

 opaque in the photographs (Fig. 11.15). 



Function of the Small Intestine. Food comes into the intestine in jets of 

 fluid projected through the relaxed circular muscle that forms the pyloric 

 valve. In doing so, it is shifted from the highly acid environment of the 

 stomach into an alkaline environment in which different digestive enzymes 

 can work. The small intestine is the most important region of the digestive 

 tract, the one where food is treated by the versatile pancreatic and intestinal 

 juices (Fig. 11.17). It is lined with millions of motile villi through which 

 almost all absorption of food occurs (Fig. 11.19). The intestinal tract con- 

 sists of the small intestine, much longer (human, about 20 feet) if not as 

 large around as the large intestine. The first part of the small intestine is the 



