THE ELEMENTARY TISSUES. 45 



iract where, by virtue of the ferments they contain, they prepare 

 the food for absorption. Another example of a secretory gland is 

 furnished by the sebaceous glands of the skin, which produce an 

 oily substance serving to keep the epidermis upon which it is 

 discharged soft and pliable. 



In the secretory glands the cells of the functional epithelium 

 elaborate within their bodies the substances necessary to give the 

 glandular secretion its peculiar and useful characters. These sub- 

 stances accumulate within the cells, where they are stored until 

 required, when they are discharged into the secretion. While in 

 the stored condition within the cells these substances may have a 

 different chemical constitution from that which they acquire when 

 they are discharged from the cells. A simple example of this 

 chemical transformation is furnished by the liver, in the epithelial 

 cells of which carbohydrates are stored as glycogen, to be liberated 

 as a closely related chemical substance, glucose. In like manner 

 the ferments stored in the epithelial cells of the digestive glands 

 are not fully formed while in that situation, but exist in states 

 known as " zymogens," from which the potent ferment appears to 

 be readily formed when the cells are called upon to furnish it. 



It is apparent, then, that the elementary tissue, epithelium, can- 

 not have the same microscopical structure in all the situations in 

 which it is found ; but, notwithstanding these variations, wherever 

 epithelium occurs it presents certain general structural peculiarities 

 which are constant and which distinguish it from the other element- 

 ary tissues. Similarly, each of the other elementary tissues pre- 

 sents variations in the details of its structure in different situations, 

 but always retains certain general structural characteristics dis- 

 tinguishing it from all the other elementary tissues. It is the first 

 task of the student of histology to learn to recognize and identify 

 these elementary tissues wherever they occur and however they may 

 vary from the type which is first presented to him for study. 



In the following chapters an attempt is made to give the student 

 an idea of the essential structure of the elementary tissues, so that 

 he may recognize them in specimens which he examines with the 

 microscope. For this purpose they have been arranged in the 

 order of their structural simplicity. 



When examining a specimen under the microscope with a view 

 to recognizing the elementary tissues it contains, the student should 

 habitually ask himself the following questions : (1) What are the 



