62 NORMAL HISTOLOGY. 



arranged in a single layer lining the tubule. If the gland is to be 

 compound, the solid column of cells branches within the tissues, and 

 then the lumina of the different portions are formed, the epithelium 

 in the different parts becoming differentiated as specialization of 

 function develops. 



The foregoing general description of the structure of secreting 

 glands applies to those glands which have a purely secretory func- 

 tion, discharging the products of their activities upon some free 

 surface, such as the skin or a mucous membrane. There are other 

 glandular organs which perform more complicated functions and 

 the structure of which deviates from that of the simpler glands. 

 Examples of these are furnished by the liver and kidney, the struct- 

 ures of which must be deferred to a subsequent chapter. Other 

 exceptions are exemplified in the thyroid body and other " duct- 

 less " glands, which discharge no secretion into a viscus or upon a 

 free surface, but which have an alveolar structure similar to an 

 ordinary secreting gland. These alveoli do not communicate with 

 ducts, which are wanting ; but whatever products they may con- 

 tribute to the whole organism are apparently discharged into the 

 circulating fluids of the body by a process of absorption similar to 

 that through which the glandular epithelium obtains its materials 

 from those fluids, or by a direct discharge into the lymphatics. (See 

 chapter on Ductless glands.) This process is indicated by the term 

 " internal secretion/ 7 and is probably of commoner occurrence than 

 is usually supposed. In fact, it but represents a special interpretation 

 of the phenomena of interchange of material that is constantly going 

 on between all the cells of the body and its circulating fluids. 



Epithelium is developed from the epiderm or hypoderm ; never 

 from the mesoderm. In this respect, as well as in its functional 

 r6le, it differs from endothelium. 



