CHAPTER FOUR 



Organs and 

 tissues 



Epithelial 

 tissue 



THE TISSUES 



1. THE animal body consists of more or less dis- 

 tinguishable parts or organs, having characteristic func- 

 tions. These organs are made up of tissues, which are 

 aggregates of cells of particular kinds. As we survey 

 the different groups of animals, we observe that the 

 organs essentially correspond throughout long series. 

 Thus even a fish has eyes, nostrils, and mouth corre- 

 sponding with those of man. Coming to the tissues, we 

 observe even closer similarities, and are obliged to con- 

 clude that the kinds of tissue were mostly evolved quite 

 early in the history of life. In spite of the astounding 

 diversity in the form of living beings, the hundreds of 

 thousands of species, the materials of which they are 

 made show comparatively little diversification. An 

 enumeration of all the known types of tissue does not 

 require much space. It is, of course, true that the 

 similar tissues of diverse animals are not exactly alike ; 

 but they are of the same general character and behave 

 in analogous ways, so that we classify them under gen- 

 eral headings, and find that one description will suffice 

 to indicate their main features. The following account 

 is based primarily on the tissues of man. 



2. Approaching the animal from the outside, we 

 meet first with the epithelium. This may be defined as 

 surface tissue, but the surfaces which it covers may be 

 external or internal. The outer covering or skin is con- 

 tinuous with the more delicate lining of the mouth, and 

 that in turn with the surface of the windpipe and gullet. 

 The epithelium may consist of a single layer of cells, as 

 in the intestine, or of many layers, as in the skin. The 

 absorbent surface of the intestine is necessarily thin ; 



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