54 GENERAL CONCEPTS 



of the vertebrate body are readily recognized when examined micro- 

 scopically. Certain tissues are composed of nonliving cell products in 

 addition to the cells; connective tissue contains many fibers in addition 

 to the fibroblasts or connective tissue cells, and bone and cartilage are 

 made largely of proteins and salts secreted by the bone or cartilage cells. 



The cells of a multicellular animal such as man may be classified 

 in six major groups, each of which has several subgroups. These are 

 epithelial, connective, muscular, blood, nervous and reproductive tissues. 



Epithelial Tissues. Epithelial tissues are composed of cells which 

 form a compact, continuous layer or sheet covering the surface of the 

 body or lining cavities within the body. There is usually a noncellular 

 basement membrane underlying the sheet of epithelial cells. The 

 epithelial cells in the skin of vertebrates are usually connected by small 

 protoplasmic processes or bridges. The epithelia of the body protect 

 the underlying cells from mechanical injury, from harmful chemicals 

 and bacteria, and from desiccation. The epithelial lining of the diges- 

 tive tract absorbs water and nutrients for use in the body. The lining 

 of the digestive tract and a variety of other epithelia produce and give 

 off a wide spectrum of substances, some of which are ui>ed elsewhere in 

 the body, and some of which are waste products which must be elim- 

 inated. Since the entire body is covered by an epithelium, all of the 

 sensory stimuli must pass through some epithelium to reach the specific 

 receptors for those stimuli. The functions of epithelia are thus protec- 

 tion, absorption, secretion and sensation. The lining of the digestive 

 tract, windpipe, lungs, kidney tubules and urinary bladder, and the 

 outer layer of the skin are some familiar examples of epithelial tissues. 



The cells in epithelial tissues may be flat, cuboidal or columnar in 

 shape, they may be arranged in a single layer or in many layers, and 

 they may have fine protoplasmic hairs or cilia on the free surface. On 

 the basis of these structural characteristics epithelia are subdivided 

 into the following groups. 



Squamous epithelium is made of thin flattened cells the shape of 

 flagstones or tiles (Fig. 3.11). It is found on the surface of the skin and 

 the lining of the mouth, esophagus and vagina. The endothelium lining 

 the cavity of blood vessels and the mesothelium lining the coelom are 

 squamous epithelia. In the lower animals the skin is usually covered 

 with a single layer of squamous epithelium, but in man and the higher 

 animals the outer layer of the skin consists of stratified squamous epi- 

 thelium, made of several layers of these flat cells. 



The kidney tubules are lined with cuboidal epithelium, made of 

 cells that are cube-shaped and look like dice (Fig. 3.11). Many other 

 parts of the body, such as the stomach and intestines, are lined by cells 

 that are taller than they are wide. An epithelium composed of such 

 elongated, pillarlike cells is known as columnar epithelium (Fig. 3.11). 

 Columnar epithelium may be simple, consisting of a single layer of 

 cells, or stratified, composed of several layers of cells. 



Either cuboidal or columnar epithelial cells may have cilia on their 

 free surface. Ciliated cuboidal epithelium is found in the sperm ducts 

 of earthworms and other animals and ciliated columnar epithelium lines 



