too much black humor, or bile. Physicians who accepted this interpretation 

 treated their patients chiefly by trying to increase or diminish the amount of 

 one or the other of these humors, so as to restore the balance. 



Chemical Foundations Today we know that the chemical processes 

 in living protoplasm are accelerated or retarded in various ways. The heart- 

 beat, for example, is accelerated by a slight increase of carbon dioxide in the 

 blood (see page 196). 



The relation between the behavior of an organ and the chemical condi- 

 tions is illustrated in certain experiments by the American biologist Jacques 

 Loeb (1859-1924). When Loeb placed strips of a turtle's heart in dishes of 

 salt solution resembling the lymph of the animal, they continued to con- 

 tract and expand regularly. When he added small quantities of various salts 

 to the different dishes, some of the strips beat faster, some slower. These 

 strips of heart continued to beat for weeks after the rest of the animal had 

 been cut up, some of it destroyed, and all of it "dead". 



Chemical Factories Metabolism itself results in various kinds of sub- 

 stances, such as carbon dioxide and water, urea and lactic acid, and other 

 waste products. In addition to these oxidation products, every kind of cell 

 produces various special substances. Do not some of these substances in- 

 fluence the metabolism of other cells ? Indeed, we have already discovered 

 that very many of the natural "poisons" and "drugs" are themselves a result 

 of plant and animal metabolism. 



Glands We have seen that in the digestion of food several special 

 fluids take part — the saliva, the gastric juice, the bile, the pancreatic solu- 

 tion (see page 169). Any organ that produces a specific substance is called 

 a gland. 



The glands are richly supplied with blood vessels. They commonly dis- 

 charge their special products on surfaces lining small cavities or tubes (see 

 illustration, p. 170). The specific product of most glands is secreted or dis- 

 charged through a special duct or tubule. 



In the middle of the last century a famous French physiologist, Claude 

 Bernard (1813-1878), discovered that the carbohydrate reserves in the liver 

 get directly into the blood-stream circulating through that organ. That is to 

 say, a substance can get out of an organ without passing through a special 

 duct. It was already known that waste substances are carried off by the 

 blood. But Bernard was impressed by the fact that special and usable prod- 

 ucts get directly into the blood-stream from the cells in which they are located 

 or formed. That was the beginning of the idea of ductless glands, as we 

 now think of these structures. 



Ductless Glands What makes the pancreatic juice come into the in- 

 testine when food from the stomach arrives there? What makes bile come 

 from the gall bladder into the intestine just when the food is ready for it 



302 



