58 HISTOLOGY 



glands, the liver, and the gastric glands. There are apparently no secre- 

 tory canals in any mucous gland, and they have not been found in the 

 duodenal, intestinal, uterine and thyreoid glands, the kidney or the 

 hypophysis. 



The ducts have a clear-cut lumen and are typically lined with a very 

 regular epithelium, showing distinct cell boundaries. The cells usually 

 do not contain the rods, granules or vacuoles characteristic of secreting 

 protoplasm, and the nuclei are not crowded to the base of the cells. 

 In some cases, however, the ducts contain mucous cells, and in the sali- 

 vary glands a specialized portion of the ducts is believed to discharge 

 salts into the secretion as it passes through them. In such a gland 

 (Fig. 47, D) the duct, as it leaves the end pieces, consists of simple flat 

 epithelium. This intercalated duct gives place to the secretory duct which 

 is lined with columnar epithelium, having basal rows of granules. The 

 outer excretory portion consists of simple or stratified non-glandular 

 epithelium. 



The end pieces of the glands, as already noted, vary in shape from 

 saccular to tubular. Usually a minute dissection or a reconstruction is 

 necessary to determine what the shape may be. A round termination 

 is called an acinus (Latin, a grape or berry) or an alveolus (Latin, a trough 

 or tray). These terms are often used interchangeably. The elongated 

 forms are called tubules. 



During the development of the thyreoid gland the duct becomes 

 obliterated, so that the secretion within the end pieces cannot escape. 

 The end pieces become closed epithelial sacs, known as follicles (Latin, 

 folliculus, a leather bag, shell, or husk). In addition to the material 

 enclosed within the follicles, the thyreoid gland secretes substances which 

 are taken up by the surrounding blood vessels and lymphatic vessels. 

 Secretions of this sort are called internal secretions. 



The epithelioid glands are masses or cords of cells which produce 

 internal secretions only. They are never provided with a duct or lumen, 

 although in some cases their cells arise from the wall of an epithelial tube. 

 They are closely related to the glands with obliterated ducts. 



Finally there are glands which produce cells and are therefore called 

 cytogenic glands. These include the ovary and testis, which are epithelial 

 structures consisting of follicles and tubules respectively. They produce 

 the ova and spermatozoa. The other cytogenic glands are non-epithelial 

 bodies which produce various forms of blood corpuscles. They will be 

 considered in a later chapter. 



The classification of glands, as presented in the preceding paragraphs, 

 is summarized in the following table: 



