THE EPIDERMIS. 



29 



Fig. 33. — Various forms of 

 ciliated epithelium. 



The simplest unstratified pavement epithelium belongs in 

 most, but perhaps not all its occurrences, to the endothelium. 

 We find it in this way on the surface of 

 the serous sacs, on the posterior wall of 

 the cornea of the eye, over the lateral 

 surfaces of the synovial capsules of the 

 joints. The same endothelia are met 

 .with on the inner surface of the cardiac 

 cavities and the vessels. 



These cells, very thin lamellae, appear 

 sometimes broad and short (Fig. 20, a), 

 on the serous membranes, again very narrow and long (5) on 

 the inner surface of the arteries. The endothelium of the veins 

 has a median character. 



A larger blood-vessel is a complicated thing. In proportion 

 as we descend to the smaller and still smaller branches, one 

 outer layer after the other disappears from this complicated 

 structure, and at last only the inner- 

 most endothelial layer remains. Large 

 cells with lapped edges, and — induced 

 by the position of the vascular tube 

 — now much more strongly incurvated 

 and in close connection, constitute the 

 walls of the capillary (Fig. 21). The 

 lymphatic vessels are also formed in 

 the same manner, though their finest 

 canals — and they occur in immense 

 numbers throughout the body — show 

 the outer surfaces of these endothelial 

 cells grown into an intimate connection 



with the neighboring tissue, so that one might here speak of 

 lacunae. 



The terminal respiratory portions of the lungs, the air vesi- 

 cles or alveoli, have a layer of simple flattened epithelium 

 which does not belong to the endothelium. 



We pass over the others at present. 



An interesting variety is found at the outer surface of the 



Fig. 34. — Endothelial cells after 

 treatment with nitrate of silver. 



