GENERAL ZOOLOGY 



Fig. 3.14. Diagrams of glands. .4, unicellular glands; the one at the left is shown extending 

 below the surface layer of cells. S, a group of gland cells that remain in the surface layer. 

 C, a simple alveolar gland. D, a simple tubular gland. E, a compound tubular gland. F, 

 a compound alveolar gland. 



receive abundant nourishment and consequently divide and replenish the 

 outer layers, which die and are cast off. Stratified squamous epithelium is 

 found in the outer layer, or epidermis, of the skin (Fig. 3.25, p. 83) and in 

 the lining of the nasal and mouth cavities, pharynx, and esophagus of many 

 vertebrates. In simple columnar epithelium, such as that lining most of the 

 digestive tract, the cells are longer than they are wide and are arranged side 

 by side (Fig. 3. 13/)). Stratified columnar epithelium is not common, but a 

 modified type is found lining the trachea (Fig. 3.\3E). Columnar epithelial 

 cells sometimes have their free surfaces, that is, the surfaces exposed to the 

 cavity they line, covered with cilia, which are very delicate cytoplasmic 

 processes (Fig. 3.13^ and F). The cilia are vibratile, and their motion 

 removes any foreign materials from the surfaces or creates currents in adjacent 

 fluids. The epithelium of the air passages in higher vertebrates and of the 

 roof of the frog's mouth is an example of this variation. In the iris and retina 

 of the eye (Fig. 4.3, p. 89, and Fig. 4.14, p. 106), epithelial cells contain 

 pigment granules and are known as pigmented epithelium. Glands are 

 organs essentially composed of simple columnar epithelium known as glandu- 

 lar or secretory epithelium (Fig. 3.13G). These cells have the capacity to 

 synthesize substances, such as enzymes or hormones, that must be present in 

 order for specific events to occur. Such substances are called secretions. 

 Glands may be unicellular, as the goblet cells of the intestinal epithelium 

 (Fig. 3.13//), or multicellular (Fig. 3.14). All glands begin their formation 

 on surfaces of the body and usually sink below the surface. In doing so they 

 may form simple tubular glands or become flask-shaped to form simple alveolar 

 glands, such as are found in the skin of the frog. Both types of simple glands 



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