EPIDERMIS 509 



surface is worn away by friction or falls off in thin flakes, and it 

 is continually renewed from below. We have seen above (p. 506) 

 that there are different types of epidermis and that these maintain 

 their nature after transplantation to another part of the body. 

 Two of the most striking are those of the sole of the foot, where 

 the stratum corneum is extremely thick and the mitotic activity 

 of the Malpighian stratum great, and the cornea, where the cells 

 are completely transparent and there is no flaking from the surface. 



A similar stratified epithelium lines the mouth, and keratinised 

 cells for examination under the microscope can always be 

 obtained by scraping the inside of the cheek with the handle of 

 a scalpel. A different type lines the trachea and bronchi. Several 

 layers of closely packed cells have on their outer layer a columnar 

 layer which is ciliated (Fig. 394). 



Transitional epithelium is found only in the urinary passages, 

 such as the ureter, bladder, and urethra. The inner cells are not 

 closely packed, so that they can slip over one another and the 

 membrane is easily stretched ; this is especially important in 

 the bladder. The surface cells are somewhat flattened, and often 

 have two nuclei each : there is a thin cuticle (Fig. 394). 



GLANDS 



A good example of a simple columnar epithelium is that which 

 lines the intestine. There is a single layer of cells, roughly 

 hexagonal in cross section, and with a height about two and a half 

 times their diameter (Fig. 397). Many of the cells are glandular 

 in function, and perhaps all become glandular in turn. Especially 

 conspicuous are the goblet cells, so called from their shape, which 

 secrete mucus. Both from these and from the enzyme-producing 

 zymogen cells the secretion is liberated at the free border of the 

 cell. In some other glands, such as those which produce milk 

 in the breast, the border partially breaks down. In both types 

 the cell goes through repeated cycles of activity, so that although 

 the discharge of a secretion by a gland may be continuous, that 

 by each individual cell is intermittent (Fig. 396). In the sebaceous 

 glands of the skin the whole cell disintegrates in order to liberate 

 its secretion. Although glandular epithelium generally remains a 

 single layer it often becomes invaginated from the surface so 

 that, in association with connective tissue, blood vessels and so 

 on. it forms a gland. According to the type of branching and 



