THE CHEMICAL BASIS OF THE CELL. 19 



which includes the process by which the digested food is built up 

 into the protoplasm of the cells, tor assimilation, and that by 

 which the resulting substances are broken down again, or disas- 

 similation. It is by these processes that the energy of life is set 

 free; the energy by which the tissues perform their functions, 

 and which appears as body heat. Every cell in the animal body 

 is therefore a seat of energy production, and at the same time 

 each is a machine for converting this energy into some definite 

 form of work. In this regard the animal machine is quite unlike 

 a steam engine, where energy liberation occurs in the furnace, 

 but conversion of this to movement occurs in the pistons. The 

 furnace and the machinery of the animal body are part and par- 

 cel of the same structures, and the digestive, circulatory, respira- 

 tory and excretory systems are more highly specialized for 

 the. purpose of transporting fuel, the oxygen to burn it and the 

 gases produced by its combustion to and from the Hying cell. 

 These processes of assimilation and disassimilation constitute the 

 study of metabolism, the practical side of which is included in 

 the science of nutrition. 



The Physico-Chemical Basis of Life. 



With the object of ascertaining to what extent the known laws 

 of physics and chemistry can explain the fundamental processes 

 that are common to all cells, we must make ourselves familiar, 

 first of all, with the chemical and physical nature of the constitu- 

 ents of the cell, and secondly with the physico-chemical laws 

 which govern the reactions that take place between these con- 

 stituents. The same laws will control the reactions which take 

 place in the juices secreted by cells; for example, in the blood 

 and in the secretions, such as the saliva. 



The Chemical Basis of Animal .Tissues. Certain substances 

 are found in every living cell and in approximately equal quan- 

 tities ; hence these may be considered the primary constituents of 

 protoplasm. In general they consist of the proteins, lipoids, in- 

 organic salts, water, and probably the carbohydrates. Protoplasm 

 is the substance composed of these primary constituents. By its 



