34 HISTOLOGY. 



central position and the protoplasm refills the cell now greatly reduced in 

 size. Most gland cells are not destroyed by the discharge of secretion, 

 but may repeat the process several times. In the sebaceous glands, 

 however, cells and secretion are cast off together, and many of the mucus- 

 producing goblet cells, such as have just been described, are thought to 

 perish after once filling with secretion. In the large intestine, goblet cells 

 are formed near the bottom of tubular depressions in a simple columnar 

 epithelium, Fig. 36. By the addition of new cells below them, they are 

 pushed toward the outlet of the tube where the oldest cells are found. 

 Mucus is discharged while its formation continues. For a time the secre- 

 tion develops faster than it is discharged, so that it accumulates within the 

 cell (Fig. 36, 2), but later, as elimination exceeds production, the cell 

 becomes emptied and dies (Fig. 36, 4). In stratified epithelium, mucus 

 may be formed in the deeper cells, but it cannot be discharged until these 

 have reached the surface. 



THE DESCRIPTION OF AN EPITHELIUM. 



In describing an epithelium the student should record its origin if it 

 is remembered, and should note from observation, first, the number of 

 layers (whether the epithelium is simple or stratified; in the latter case, 

 the number of strata) ; second, the shapes of the cells (columnar, cuboidal, 

 or flat, and in a stratified epithelium the layers, basal or superficial, in 

 which such shapes occur) ; finally, the special structures should be sought, 

 including basement membranes, intercellular bridges, terminal bars, 

 striated, brush, or ciliated borders, and forms of secretion within the 

 protoplasm. A detailed description of nucleus and protoplasm should 

 be given of such epithelial cells as are of special importance. 



THE NATURE AND CLASSIFICATION OF GLANDS. 



A preliminary description of glands may be inserted at this point, 

 since glands in the strictest sense are groups of such secreting epithelial 

 cells as have just been described. Two other classes of structures are called 

 glands, however. In one of these, cells instead of secretions are formed 

 and set free. Cell-producing glands are called specifically cytogenic 

 glands. These include, first, the ovary and testis which produce sexual 

 cells; and, second, the lymph glands, haemolymph glands, spleen, and 

 red bone marrow, all of which produce blood corpuscles. Tissue similar 

 to that of the lymph glands when found in a diffuse form is not called glan- 

 dular, but merely lymphoid tissue. The term gland, as here employed, 

 suggests a well-defined, macroscopic mass of cell-producing tissue, epithe- 

 lial in the sexual glands, and non-epithelial in the lymphoid group. 



