58 NORMAL HISTOLOGY. 



tissue is displayed chiefly in an active multiplication and growth of 

 its cells. This is especially true in the stratified variety, where pro- 

 tection is provided by a constantly renewed supply of cells. In 

 other cases the substances received by the cells are elaborated into 

 definite compounds destined to form the essential constituents of a 

 secretion. This secretory function of epithelium is an extremely 

 important one, and for its performance that tissue is usually ar- 

 ranged in a special structure or organ, called a gland. A brief state- 

 ment of the general characters and classification of these organs 

 may here appropriately find a place. 



Secreting Glands. The simplest type of secreting structure con- 

 sists of a surface covered with a layer of epithelium, the cells of which 

 are endowed with the power of elaborating a secretion and discharg- 

 ing it upon their free surfaces (Fig. 32, 6). The tissues supporting 

 the epithelium belong to the connective tissues, and are fibrous in 

 character and well provided with bloodvessels, lymphatics, and 

 nerves. These bring to the epithelium the substances necessary for 

 its nourishment and work, and place its activities under the control 

 of the nervous system. Between the epithelium and the fibrous 

 tissue supporting it there is frequently a thin membranous layer of 

 tissue that often appears quite homogeneous, evidently belongs to the 

 connective tissues, and has received the name of "basement-mem- 

 brane." This appears to offer a smooth surface for the attachment 

 of the epithelial cells, which receive their nourishing fluids through it. 



The epithelial surfaces of many of the mucous membranes are 

 examples of the foregoing simple secreting structure. The secretory 

 function is here of use as an adjunct to the protective function 

 assigned to the epithelial covering, and the quantity of secretion is 

 but slight under normal conditions. Where the volume of secre- 

 tion required is considerable some provision for an increase in the 

 extent of secreting surface is necessary. This may be accomplished 

 by an invagination of that surface, which then forms the lining of 

 one or more tubes or sacs, into which the secretion furnished by the 

 epithelial cells is discharged. Such an arrangement of the tissues 

 constitutes a gland, and it is evident that these may be arranged 

 into groups or classes according to whether the secreting surface 

 forms a single tube or sac, or several such tubes or sacs, uniting to 

 form a single gland. Thus, there may be simple or compound tubular 

 glands, or simple or compound saccular glands. Whether the deeper 

 portions of the gland have a tubular or saccular structure, the secre- 



