28 PRINCIPLES OF VETERINARY SCIENCE 



epiblast and hypoblast of the embryo and are found on or near 

 surfaces of the body. All are capable of rapid replacement 

 when injured or destroyed. This makes them well fitted to 

 cover surfaces exposed to injury. 



There are four kinds of epithelial tissue, namely: (1) squamous 

 epithelium; (2) columnar epithelium; (3) secreting epithelium; 

 (4) ciliated epithelium. 



1. Squamous epithelium is primarily a protective tissue and is 

 composed of flat, scale-like cells. Its cells may be in single layers, 

 like the cells lining the air sacs of the lungs; or stratified and com-, 

 posed of several layers piled one upon the other as are the cells of 

 the skin, hair, horns, and hoofs (see Fig. 2). The upper layers 

 are farthest from the nutrition supply, and so lose their sensitive- 

 ness and become hard and act as good protective agents. Poisons 

 cannot readily pass through, and chemicals do not act easily on, 

 intact epithelium. Its function is chiefly to protect the sensitive 

 structures beneath, like the "quick" under the hoofs. 



2. Columnar epithelium derives its name from the shape and 

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

 It is found lining 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 secretions of the body, and is found composing the various 

 secretory glands, all of which are epithelial outgrowths. The 

 simplest form of gland is the tubular (see Fig. 28). Branched 

 and compound glands are more complex. Depending upon the 

 substances produced, secreting epithelium may be: 



(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 lubricat- 

 ing 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 addi- 

 tion of water it swells and forms a transparent glairy mass. Its 

 function is to act as a lubricant. 



(6) Zymin-secreting epithelium, which forms the various 

 juices in the mouth, stomach, and bowels that act upon and 

 digest the food. The activity of the secretions is due to the 



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