GIL IIL] 



EPITHELIUM 



21 



CLASS 1. Simple epithelium; that is, an epithelium consisting 

 of only one layer of cells. Its subgroups are : 



a. Pavement epithelium. This consists of a layer of thin cells 

 arranged in the form of an accurately fitting mosaic ; this is typically 

 seen in the epithelium that lines the air-sacs of the lungs (fig. 25). 

 The endothelium found in the interior of 

 the blood and lymph vessels and serous 

 sacs is very similar in structure, but 

 differs from other epithelia in being the 

 only one of mesoblastic origin (fig. 26). 



b. ^Cubical and columnar epithelium. 

 Here the cells, as their names imply, are 

 thicker. Cubical epithelium is found in 

 the alveoli of the thyroid, in the tubules 

 of the testis, and in the ducts of many 

 glands. Columnar epithelium lines the 

 alimentary canal from the stomach to 

 the anus. 



The four figures (figs. 27-30) present 

 the very typical columnar cells, each 

 with a bright striated border, which 

 are found lining the intestine. Fig. 28 

 shows how they are arranged on the 

 surface of a villus, one of the numerous 

 little projections found in the small in- 

 testine. The gaps seen there are due to 

 the formation of what are called goblet 

 cells. In some of the columnar cells a 

 formation of granules occurs ; these con- 

 sist of a substance called mucigen ; these 

 run together and are discharged from the 

 cell as a brightly refracting globule of 

 mucin, leaving the cell with open mouth 

 like a goblet, the nucleus being sur- 

 rounded by the remains of the protoplasm in the narrow stem 

 (fig. 30). This transformation is a normal process continually going 

 on throughout life, the discharged mucin being the chief constituent 

 of phlegm or mucus. The cells themselves may recover their original 

 shape after discharge and repeat the process later on. 



c. Ciliated epithelium ; this form of epithelium presents so many 

 points of physiological interest, that a separate section will be 

 devoted to it later in this chapter. 



CLASS 2. Compound ^Epithelium; that is, an epithelium con- 

 sisting of more than one layer of cells. It contains two subgroups, 

 a. Transitional epithelium found lining the bladder and ureters. 



FIG. 26. Surface view of an artery from 

 the mesentery of a frog, ensheathed 

 in a peri vascular lymphatic vessel, 

 a, The artery, with its circular 

 muscular coat (media) indicated by 

 broad transverse markings, with 

 an indication of the adventitia out- 

 side. I, Lymphatic vessel ; its Avail 

 is a simple endothelial membrane. 

 (Klein and Noble Smith.) 



