34 



EPITHELIAL TISSUES 



secreting glands salivary glands, kidney, and liver. In many epithelia 



secretion is a subsidiary function, protection being the primary purpose. 



In all epithelia a cement substance is present between the cells. 



This becomes especially abundant and dense between the distal ends of 



the cells of columnar epithelium, and 

 is here known as terminal bars (Fig. 

 40). Cement substance has the pe- 

 culiar property of precipitating sil- 

 ver nitrate from solutions, which 

 turns black on exposure to sunlight. 

 This furnishes an especially favor- 

 able technic for demonstrating cell 

 boundaries. All epithelia, simple or 

 stratified, rest upon a homogeneous 

 basement membrane or membrana 

 propria, frequently a product of the 

 cells themselves but occasionally of 

 connective tissue origin, and a sub- 

 jacent connective tissue supporting 

 membrane or tunica propria (or 

 corium). The latter only contains 



blood and lymph vessels from which the epithelial cell must draw nour- 

 ishment by process of absorption, and transfer through 'intercellular 

 bridges/ It furnishes support also for the nerve supply. We may now 

 consider briefly the usual types of simple and stratified epithelia. The 

 main facts are summarized in the appended outline : 



FIG. 40. 'TERMINAL BARS' OF CEMENT 

 SUBSTANCE AS SEEN BETWEEN THE 

 EPITHELIAL, CELLS OF A TUBULAR 

 SECRETING GLAND IN THE PYLORIC 

 REGION OF THE HUMAN STOMACH. 

 The columnar epithelium is seen in 

 profile at a; at b, the free ends of the 

 cells are seen. Hematein. X 550. 



CLASSIFICATION OF EPITHELIA 



I. SIMPLE (NON-STRATIFIED) EPITHELIA those which com- 

 pose a membrane but one cell in thickness. Epithelial cells, 

 usually spherical or polyhedral in shape, occur also en masse 

 in the form of cords or clusters. 



1. Squamous, 

 composed of 

 flattened, 

 scale-like 

 cells. 



(a) Lining closed cavities. 

 Pavement epithelium 



or (1) endotkelium; heart, arteries, capillaries, veins, 

 and lymphatic vessels. 



(2) mesothelium; serous membranes. 



(3) mesenchymal epithelium,; synovial membranes, 



bursae, and tendon sheaths, lining of the anterior 



