44 NORMAL HISTOLOGY. 



respiratory passages the conditions are different. Here the tissues 

 require protection from particles of dust that may be inhaled. For 

 this purpose the epithelial cells lining those passages are provided 

 with minute, hair-like processes, "cilia," which execute lashing move- 

 ments toward the outlets of the passages and occasion the transpor- 

 tation of substances coming into contact with them toward the 

 outer world. In the digestive tract the conditions are again differ- 

 ent. The tissues underlying the epithelial lining need protec- 

 tion from the chemical action of the fluids in the stomach and intes- 

 tine, as well as from friction with their solid contents. The cells 

 of the epithelium meet these needs by a secretion of mucus, which 

 is discharged upon the inner surfaces of the digestive organs, where 

 it serves as a protective layer and as a lubricant. 



In other situations epithelium has an excretory function, which is 

 less clearly of value in protecting its immediate surroundings, but 

 is essential for the protection of the whole organism from substances 

 which would exert an injurious effect if they were permitted to ac- 

 cumulate in the circulating fluids of the body. These substances 

 are absorbed from those fluids by epithelial cells, from which they 

 are discharged from the body either unchanged or after transforma- 

 tion into other chemical compounds. Here the most obvious prod- 

 ucts of cellular activity are of no use in the economy, and are elim- 

 inated from it; but it is not improbable that the cells which separate 

 them or their antecedents from the circulating fluids may also 

 discharge useful substances into those fluids (" internal secretion "). 

 We must not assume that the most obvious function exercised by a 

 tissue is the only service it does to the organism. 



The epithelium which carries on this eliminative function is nearly 

 always associated with other elementary tissues to form an organ, 

 called a "gland," in which the epithelium is the functionally active 

 tissue, the other tissues being subservient to it. The glands of the 

 body differ considerably in both structure and function, but in all 

 of them it is epithelium which elaborates the materials essential to 

 the formation of their normal secretions. Mention has already 

 been made of those glands which furnish secretions charged with 

 waste materials to be eliminated from the body. Such glands are 

 called excretory glands, and are exemplified by the kidney. Other 

 glands, distinguished as secretory in a restricted sense, furnish secre- 

 tions which are of service to the organism. Examples of such 

 glands are those which discharge their secretions into the alimentary 



