no 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



the hands, and thick and tough in the footpads of cats and dogs; the cells are 

 heavily cornified in fingernails, in the horns of cattle, in the hoofs of horses, 

 the outer shell of turtles, the hair of mammals, and feathers of birds. New 

 epitnelial cells are formed as others are worn out or injured. Otherwise we 

 should be walking records of our encounters — scrapes, burns, and pinches. 

 Regeneration is constantly going on in skin and its outgrowths of feathers and 

 hairs. The snake's skin comes off in one piece, the human skin in little frag- 

 ments; feathers are shed in late summer, human skin at any time. 



Different Kinds of Epithelial Cells. These are classified according to their 

 shapes — flattened, cuboidal, columnar, and arrangement in single or multiple 

 layers, simple or stratified (Table 7.1). A single layer of simple flattened (or 

 squamous) epithelium lines blood and lymph vessels including the heart (Fig. 

 7.1). Cuboidal epithelium lines the ducts of glands (Fig. 7.2). The cells of 

 columnar epithelium are tall prisms or cylinders (Figs. 7.3, 7.4). They form 

 the lining of the small intestine where they secrete digestive juices and absorb 

 the digested food. All columnar cells have polarity, that is, are different at their 

 two ends. In ciliated columnar cells the polarity is conspicuous since they bear 

 a large number of cilia only on their free surfaces. Cilia beat with rapid 

 effective and slower recovery strokes, always bending in one direction. The 

 movements travel over the surface in waves which rapidly succeed each other 

 at regular intervals. This occurs in the lining of the human trachea with the 

 stronger stroke toward the mouth. Cilia on the gills and lips of clams wave 

 particles of food toward the mouth. In the oviducts of mammals they create 

 currents which move the eggs toward the uterine cavity (Fig. 18.13). 



Stratified flattened epithelium of the skin is several layers thick; the outer 



Table 7.1 

 Forms and Functions of Epithelium 



