CHARACTERISTICS OF THE ANIMAL BODY 27 



not readily pass through, and chemicals do not act easily on, in- 

 tact epithelium. Their function is chiefly to protect the sensitive 

 structures beneath, like the "quick" under the hoofs. As a rule, 

 all epithelial tissues are capable of rapid replacement when injured 

 or destroyed. This makes them well fitted to cover surfaces ex- 

 posed to injury. 



2. Columnar epithelium derives its name from the shape and 

 arrangement of its cells, which are longer than they are broad. 

 It is found hning the stomach and intestines. The chief function 

 is to take up matter digested by these organs and to pass it on to 

 the blood. 



3. Secreting epithelium, as the name indicates, has to do with the 

 secreting tissues of the body, and is found composing the various 

 secretory glands which are outgrowths of the epithelium. The 

 simplest form of gland is the simple tubular. Branched and com- 

 pound glands are more complex. Depending upon the substances 

 produced, secretory epithelium may be either — 



(A) Mucin-secreting epithelium, which is the variety found 

 wherever mucous membranes are located, that is, in all passages 

 which communicate either directly or indirectly with the outer 

 air. It secretes a slimy substance known as mucin for lubricating 

 the mouth, stomach, intestines, and other organs. Mucin is of 

 great importance in the animal economy. When precipitated and 

 freed from water it is white and amorphous. On the addition of 

 water it swells and forms a transparent glairy mass. Its function 

 is to act as a lubricant. 



(B) Zymin-secreting epithelium forms the various juices in the 

 stomach and bowels which act upon and digest the food. The 

 activity of the secretions is due to the presence of enzymes, zymins, 

 or hormones. During the activity incidental to digestion these 

 cells give off or secrete, large quantities of enzymes. When this 

 process ceases, the cells, now in the so-called resting stage, are 

 storing up their product for future use. Carefully prepared speci- 

 mens for microscopic study clearly demonstrate that peculiar 

 granules are responsible for the action of the secretion. These 

 disappear with glandular activity. 



(C) Excreting epithelial tissues pass substances out of the body 

 which are of no further use in the animal economy, e. g., the epi- 

 thelium of the kidneys, sweat glands, and mammary glands. 

 They not only have the power of taking up material from the blood 



