INTRODUCTORY 3 



chemically simpler products. This conversion is 

 due to the presence in the saliva of "ptyalin," a 

 substance that belongs to a class of compounds 

 known as "enzymes" or "ferments." Much of the 

 chemical work of the body and this includes the 

 plant as well as the animal kingdom is due to the 

 activity of these enzymes. 



A gland, then, is an organ that has the power of I 

 taking certain materials from the blood and manu- / 

 facturing from these raw materials a product/ 

 which plays a part in the activities of the body.' 

 The salivary glands, wherein saliva is manufac- 

 tured, are an example. The gastric glands lining 

 the wall of the stomach, manufacturing gastric 

 juice for the stomach needs, are another. The 

 pancreatic and intestinal juices arising, respec- 

 tively, from the pancreas and the small intestine, 

 are still others. These examples could be multi- 

 plied. 



Ductless or endocrine glands, or glands of inter- 

 nal secretion. So far we have been considering 

 glands that have tubes or "ducts" connecting them 

 with an outer surface, such as the mouth or stom- 

 ach. Glands are known, however, that have no such 

 ducts, but that pass their products directly into the 

 blood stream. Such glands are known as "duct- 

 less" glands. Sometimes they are spoken of as 

 glands of "internal secretion," to distinguish them 

 from those glands with ducts whose secretion is 



