PROBLEM 2. The Digestive System 



the absorption of fats. But the Hver is 

 helpful in still other ways. Besides con- 

 taining gland cells which make bile it 

 has ordinary cells which serve as a store- 

 house of carbohydrates. You have read 

 that the end product of carbohydrate 

 digestion is a simple sugar, mostly glu- 

 cose. This is the kind of su^ar that the 

 cells of the body can use. However, the 

 blood cannot hold more than a small 

 amount of glucose at one time. Under 

 normal conditions, much of the suaar 

 that is absorbed into the blood leaves the 

 blood stream in the liver. Here it is 

 changed into an insoluble material similar 

 to the starch found in a plant. This in- 

 soluble substance is called glycogen (gly'- 

 ko-jen). Converting glucose into gly- 

 cogen is the opposite of digestion. An 

 enzyme brings about the change. Later, 

 as the amount of sugar in the blood de- 

 creases, the glycogen is changed back 

 into glucose and diffuses into the blood. 

 In this way the sugar concentration in 

 the blood is kept almost constant. 



The liver functions in another way. 

 It changes certain amino acids which 

 would otherwise be wasted into a useful 

 substance, glucose. On page 190 amino 

 acids were compared to "building stones" 

 as parts of protein molecules. When you 

 eat protein and eventually build up your 

 own bod\^ protein, it is much as though 

 you wrecked a number of houses and 

 then used some parts of one and other 

 parts of another to build your own new 

 house. Your new house has a special de- 

 sign which makes it impossible for you 

 to use all the parts of the house or houses 

 you wrecked. So when you eat and di- 

 gest (wreck) proteins from various ani- 

 mals and plants you get "building stones" 



Makes Food Usable 



199 



(amino acids) of different kinds. You 

 can use some for assimilation; the others 

 are changed in the liver into two sub- 

 stances. One is a nitrogen compound 

 called urea, a waste product. The other 

 is glucose which can either be oxidized 

 immediately or stored as glycogen. 



What stirs the glands to action? Secre- 

 tion in the glands must be well timed, or 

 much digestive juice will be wasted. You 

 have already read how the sight and 

 smell or even thought of food sends mes- 

 sages along the nerves to the salivary 

 glands and the gastric glands which then 

 begin to secrete actively. This goes on 

 without your thinking about it or know- 

 ing what goes on inside of you. The mere 

 presence of food against the mucous lin- 

 ing also stirs these glands to action. 



But digestive glands may be made to 

 secrete actively in still another way. At 

 the beginning of this century two Eng- 

 lish scientists performed a very interest- 

 ing and important experiment. They had 

 been led to believe that there were sub- 

 stances in the blood which stimulated 

 the pancreas to secrete. To test their 

 theory the following experiment was 

 performed. Two dogs were operated 

 upon and a large blood vessel of one dog 

 was joined to the corresponding blood 

 vessel of the second dog. In this way the 

 blood of each dog flowed through the 

 body of the other. Then one animal was 

 fed and the other was left unfed. After 

 some time, when the digested food ar- 

 rived in the small intestine of the dog, 

 there was a flow of juices in both dogs! 

 Since the only connection between the 

 two dogs was their blood vessels it 

 seemed as though something to stimulate 

 the pancreas must have been carried by 



