GENERAL STRUCTURE OF THE BODY. 13 



all of which at first appear alike. By degrees, however, 

 differentiation takes place and different groups of cells 

 assume different characteristics. Thus the various 

 tissues are gradually developed, each with a structure 

 and a function of its own, and are distributed among the 

 various organs, each organ consisting of several tissues. 

 During the process of growth and even after full growth 

 of the body is attained old cells are continually dying and 

 being replaced by new ones. 



The typical cell is circular, but through being squeezed 

 together in the tissues or for some other reason the cells 

 vary in shape in different parts, being at times hexagonal, 

 spindle-shaped, or columnar. Yet, whatever their differ- 

 ences in shape or other characteristics, they all live the 

 same sort of life. All protoplasm absorbs oxygen when 

 it comes in contact with it and in the process of combin- 

 ing with it is in part burned or oxidized, with the conse- 

 quent setting free of heat and other forms of energy and 

 the formation of carbon dioxide. So long as the body 

 is alive, therefore, whether it -is in a state of activity or of 

 rest, it is the seat of constant chemical change through- 

 out all its cells, and to these chemical changes are due all 

 the forms of energy manifested by the body. For energy 

 is never destroyed, though it may appear in a different 

 form, and the elements of the human body are so com- 

 bined that their energy may be liberated and manifested 

 in the different functions the body exhibits. 



The fundamental tissues of the body are the epithelial 

 tissues, the connective tissues, including the cartilagin- 

 ous and bony tissues, and the muscular and nervous 

 tissues. Of these the epithelial tissues serve as a protec- 

 tion to the surface of other tissues; the connective tissues 

 together form a framework for the support and general 

 protection of the other tissues; while energy is expended 

 by muscular and nervous tissue, the latter directing the 

 former in its movements. All the tissues are inter- 

 dependent and the organs work together. Besides cells 

 every tissue contains a certain amount of lifeless matter, 



