IV. STRUCTURAL UNITS OF HUMAN TISSUES — 

 HISTOLOGY. 



The study of the structural units (the cells) that make 

 up the human tissues, is called microscopic anatomy or 

 histology. The methods of microscopy make it possible 

 for us to cut very thin sections of tissue, to stain them 

 with certain dyes so as to bring out the different parts of 

 the cells, and finally, to mount them on glass slides in 

 such a way that they may be examined with the com- 

 pound microscope. This instrument, by magnifying many 

 diameters, makes it possible for us to study details of 

 structure that were otherwise invisible. The various tissues 

 that are found in the body are found to be capable of clas- 

 sification into four general classes of tissues: epithelial, 

 nervous, muscular, and connective. This classification 

 is based upon the characters of the cells of which the 

 tissues are formed. 



Epithelial tissue. — All surfaces of the body, external 

 and internal, are protected by layers of membranes. The 

 external layer we call the skin; the layer which lines the 

 mouth, digestive organs, and lungs we call the mucous 

 membrane. When we examine these membranes under 

 the microscope they are found to be composed entirely of 

 cells united in layers by a thin cementlike substance, 

 much as bricks are held together by mortar. Owing to 

 the fact that the membranes which these cells form are 

 covering or protective layers, the name of epithelial tissue 

 has been given to them, and the cells of which they are 



