CHAPTER II. 

 TISSUES. 



A tissue is a complex of similarly differentiated 

 cells and their derivatives. In all embryonic tissues 

 and some adult tissues the cell elements predominate, 

 but in cartilage and bone and many connective tis- 

 sues the cell products make up the bulk of the tissue. 

 There are four kinds of elementary tissues, (i) 

 epithelial tissue; (2) Supporting tissue; (3) Muscular 

 tissue; (4) Nerve tissue. 



I. EPITHELIAL TISSUE. 



Epithelium lines surfaces, external and internal, 

 and forms the secreting cells of glands. (See table, 

 P a ge 33.) The cells of this tissue are derived from 

 any one of the three germ layers. Blood and lymph 

 vessels do not penetrate between the epithelial cells, 

 but nerve fibers enter the deeper strata and end in 

 minute varicosities that lie in contact with many of 

 the cells. The cells have a regular form, a thin cell 

 wall, and a distinct nucleus that is rich in chromatin 

 and therefore stains easily with hematoxylin. They 

 usually secrete a cement found between adjacent cells 

 which serves the purpose of holding each cell firmly in 

 place. The cell wall is usually smooth, but in certain 

 places the lateral walls develop many short, minute 

 processes (prickles) that meet like processes from 

 adjacent cells, forming intercellular bridges, and be- 

 tween the latter are found intercellular spaces filled 

 with nourishing fluid for the individual cells. The 



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