22 THE HUMAN BODY 



portion of the protoplasm which may become eventually an inde- 

 pendent organism, like the parent; (6) growth, the property, based 

 on assimilation, of increasing in size by the building up of the body 

 protoplasm faster than it is broken down. 



The Body as a Machine. Dissimilation. All living things, 

 from the simplest one-celled organism to man himself, obey the 

 mechanical law of the conservation of energy. By this we mean 

 that whenever energy is manifested, as in motion, heat production, 

 or any other form of activity, an equivalent amount disappears 

 from some antecedent source. The source of bodily energy is 

 chemical, in animals being derived, ultimately, from the food. All 

 cell activities involve the expenditure of energy. All cells, there- 

 fore, require to be fed. Energy is obtained in animal cells through 

 the breakdown of the complex chemical substances of which food 

 is composed into simpler ones. This breaking^down process is 

 described as dissimilation, the opposite of the building-up process, 

 assimilation. In living cells the two processes, assimilation and 

 dissimilation, go on side by side, and under ordinary conditions 

 exactly balance, so there is neither gain nor loss of cell substance. 

 Since dissimilation is the process by which the Body obtains 

 energy for its various activities we shall have occasion to study itb 

 manifestations in detail as the different phases of bodily function 

 are considered. 



Cell Growth. All cells during their early life possess the power 

 of growth, or in terms of their chemical activities are able to assim- 

 ilate faster than they dissimilate. The materials that are to be 

 assimilated have to enter the cell through its surface membrane, 

 and obviously, if there is no shortage of food, the larger the surface 

 the more can enter. The processes of dissimilation, going on in- 

 side the cell are, on the other hand, relatively independent of the 

 surface, being determined rather by the amount or mass of proto- 

 plasm making up the cell. Now a little consideration will make 

 clear to us that the smaller an object is the larger is its relative 

 surface. This we can demonstrate by placing two bricks together 

 to make one block. The dimensions of the block will be 4"x4" 

 X8", and the surface area 160 sq. inches. If now we separate the 

 two bricks each will have exactly half the mass of the former block 

 but more than half the surface, the total surface area of a single 

 brick being 112 sq. inches. Thinking now of cells instead of bricks 



