CELLS AND TISSUES 



55 



Columnar 



Glandular 



Figure 3.1 1 . Diagram o£ the types of epithelial tissue and their location in the body. 



the ducts of the respiratory system of man and other air-breathing 

 vertebrates. The rhytlimic, concerted beating of the cilia moves solici 

 particles in one direction through the ducts. Epithelial cells, usually 

 columnar ones, may be specialized to receive stimuli. The groups of 

 cells in the taste buds of the tongue or the olfactory epithelium in the 

 nose are examples of sensory epithelium. Columnar or cuboidal epithelia 

 may also be specialized for secreting certain products such as milk, wax, 

 saliva, perspiration or mucus. The outer epithelium of most worms 

 secretes a thin, continuous, noncellular protective layer, called the 

 cuticle, which covers the entire body. Insects, spiders, crabs and other 

 arthropods secrete a cuticle Avhich may be quite thick and strengthened 

 with deposits of chitin and salts. The hard protective shell of oysters 

 and snails, composed of calcium carbonate, is secreted by epithelial cells 

 in the mantle of these animals. 



Connective Tissues. The connective tissues— bone, cartilage, ten- 

 dons, ligaments, fibrous connective tissue and adipose tissue— support 

 and bind together the other tissues and organs. Connective tissue cells 

 characteristically secrete a nonliving material called the matrix, and 

 the nature and function of each connective tissue is determined pri- 

 marily by the nature of this intercellular matrix. The actual connective 

 tissue cells may form only a small and inconspicuous part of the tissue. 

 It is the matrix, rather than the connective tissue cells themselves, which 

 does the actual connecting and supporting. 



Fibrous connective tissue consists of a thick, interlacing, matted net- 

 work of fibers in which are distributed the cells that secreted the fibers 

 (Fig. 3.12). There are three types of fibrous connective tissue, widely 

 distributed throughout the body, which bind skin to muscle, muscle to 

 bone, and so on. These include very delicate reticular fibers, thick, 



