CELLS AND TISSUES 33 



52. The Tissues. Sets of similar cells are grouped to- 

 gether to form tissues. The cells may develop long 

 branches, called processes or fibers, and may deposit mate- 

 rial between adjoining cells called intercellular substance. 

 A tissue consists of a mass of cells built up together, 

 along with all of their processes and intercellular substance. 

 A part of the body which has a definite work to do is called 

 an organ, and each organ consists of several tissues. The 

 tissues are of many kinds. In order to understand them 

 clearly and to remember them distinctly, we must learn 

 them according to their uses in the different organs. 



53. What are the chief uses or purposes of our bodies ? 

 If we consider our life, we shall at once say that our bodies 

 enable us to perceive, to feel, to think, on the one hand, 

 and to work, to execute, to exert energy, on the other. 

 The powers of thinking and doing have two tissues allotted 

 to them in the animal economy, and these two tissues have 

 been named by physiologists, the master tissues, while 

 the remaining tissues, to which are intrusted the protec- 

 tion, support, and renewal of the master tissues, are called 

 the supporting tissues. The two master tissues, or those 

 that have to do with controlling and exerting energy, are 

 called, respectively, the muscular and the nervous tissues. 

 They will be treated in a general way in the next chap- 

 ter, so that their part in the functional activity of each of 

 the various organs may be understood as they are taken 

 up in turn. Those organs the chief functions of which 

 are nervous or muscular will be studied in later chapters. 



54. In studying the definitions of the tissues and the 

 drawings showing microscopic views of them, it will be 

 well to remember that they are all developed from the 

 same elements, namely, nucleated cells. Every tissue in 

 the earlier stages is a mass of such cells packed together. 

 The cells of one tissue, when mature, resemble one another, 

 but are unlike those of other tissues. 



