Cfi. III.] 



EPITHELIUM 



25 



The general appearance presented by endothelium in serous mem- 

 branes is shown in figs. 29, 30, and 31 ; in blood-vessels in fig. 32. 



The stomata seen in some of the 

 drawings are minute openings sur- 

 rounded by more darkly staining cells, 

 which lead from serous cavities into 

 lymphatic vessels. 



Cubical, Spheroidal, and Columnar 

 Epithelium. 



In these forms of epithelium, the 

 cells are not flat, but are thick ; if they 

 approximate cubes or spheres in shape, 

 the epithelium is called cubical or 

 spheroidal respectively. Polyhedral epi- 

 thelium is found in the alveoli of secret- 

 ing glands, such as the salivary glands, 

 liver, and pancreas (see figs. 33 and 34), 

 and will be discussed at length in con- 

 nection with those organs. Cubical epi- 

 thelium is found in the alveoli of the 

 thyroid (see fig. 35), in the tubules of 

 the testis, and in the ducts of some 

 glands. 



In Columnar epithelium the Cells are FIG. 32.-Surrace view of an artery from 



tall, and form a kind of palisade or rows 

 of columns. It is found lining the in- 

 terior of the stomach and intestines, and 

 the ducts of the majority of secreting 

 glands; it forms also the layer on the 

 outer surface of the ovary. 



In the intestinal epithelium each cell has a distinct brightly 

 refracting and striated border. Fig. 36 shows two isolated cells of 

 this kind. 



The nucleus with its usual network and the vacuolated condition 

 of the protoplasm are very well seen. The attached border is narrower 

 than the free edge. Amoeboid lymph cells are found in the spaces 

 that must necessarily be left when cells of such a shape cover a 

 surface. Fig. 37 shows a row of columnar cells from the rabbit's 

 intestine. 



The next figure (fig. 38) shows the arrangement of these cells on 

 the surface of a villus, one of the numerous little projections found 

 in the small intestine. 



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 



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 wall 

 is a simple endothelial membrane. 

 (Klein and Noble Smith.) 



