NORMAL HISTOLOGY. 61 



CHAPTER VII. 



ENDOTHELIUM AND EPITHELIAL TISSUES 



Endothelium, This structure is of mesodermic origin and con- 

 sists of a single layer of very thin polyhedral cells united edge to 

 edge by a cement substance. It lines surfaces not directly ex- 

 posed to the external atmosphere, such as the surfaces of serous 

 and synovia! membranes, forming the linings of the heart, blood 

 tubes, and other organs. 



EPITHELIUM. 



Epithelium is derived from the epiderm and hypoderm of the 

 embryo. It consists of cells of various shapes, which are united by 

 cement and are devoid of blood-vessels. The important functions of 

 epithelium are protection, secretion, and elimination. The shape 

 of the cells depends upon the amount and kind of pressure exerted 

 upon them. As a rule, when first formed, they are spherical. New 

 cells are produced by karyokinesis. Blood vessels being absent, 

 nourishment takes place by absorption. 



The following are the important varieties : 



Squamous. Simple and Stratified. 



Columnar. Simple and Stratified. 



Ciliated. Simple and Stratified. 



Modified. Goblet, Pigmented, and Transitional. 



Specialized. Glandular and N euro-epithelium. 



Squamous epithelium. This structure consists of irregular, 

 polyhedral, flattened cells, united edge to edge in the form of a pave- 

 ment. The cells, when seen on edge, are found to be somewhat bi- 

 convex. The nucleus is somewhat eccentric. Squamous epithelium 

 is found wherever surfaces are subjected to considerable friction. 

 There are two varieties viz., simple and stratified. Simple 

 squamous epithelium consists of a single layer of cells and is found 

 lining the air cells of the lungs, the capsule of the Malpighian body, 

 the descending limb of Henle's arch, parts of the brain-ventricles, 

 and a few other places. 



The stratified variety consists of several layers and is found cov- 



