34 



ELEMENTARY TISSUES. 



CHAPTER IV. 1 



STRUCTURE OF THE ELEMENTARY TISSUES. 



Epithelium. 



ONE of the simplest of the elementary structures of which 

 the human body is made up, is that which has received the 

 name of Epithelium. Composed of nucleated cells which are 

 arranged most commonly in the form of a continuous mem- 

 brane, it lines the free surfaces both of the inside and outside 

 of the body, and its varieties, with one exception, have been 

 named after the shapes which the individual cells in different 

 parts assume. Classified thus, Epithelium presents itself under 

 four principal forms, the characters of each of which are dis- 

 tinct enough in well-marked examples; but when, as frequently 

 happens, a continuous surface possesses at different parts two 

 or more different epithelia, there is a very gradual transition 

 from one to the other. 



1. The first and most common variety is the squamous or 

 tessellated epithelium (Figs. 1 and 2) which is composed of flat, 



FIG. 1. 



FIG. 2. 



FIG. 1. Fragment of epithelium from a serous membrane (peritoneum) ; magnified 

 410 diameters, a, cell; b, nucleus; c, nucleoli (Henle). 



FIG. 2. Epithelium scales from the inside of the mouth ; magnified 260 diameters 

 (Henle). 



oval, roundish, or polygonal nucleated cells, of various size, 

 arranged in one, or in many superposed layers. Arranged in 



1 The following chapter, containing aw outline-description of the 

 elementary tissues, has been inserted for the convenience of students. 

 For a much fuller and better account, the reader may be referred to 

 Dr. Sharpey's admirable descriptions in Quain's Anatomy. 



