18 PHYSIOLOGY FOR DENTAL STUDENTS. 



its own, although certain functions remain which are common 

 to all. In other words, as the organism becomes more and more 

 complex, there comes to be a division of labor on the part of the 

 cells that comprise it. The conditions are exactly like those 

 which obtain in the development of a community of men. In 

 primeval communities' there is little division of labor, every indi- 

 vidual makes his own clothes, hunts his own food, manufactures 

 and uses his own implements of war, but as civilization begins 

 to appear, certain individuals specialize as hunters and fighters, 

 others as makers of clothing, others as artisans. Although, in 

 its first stages, this division of labor may be far from absolute, 

 for every member of the community must still fight and take part 

 in the building of his hut, yet it soon tends to become more and 

 more so, until, as in the civilized communities of this twentieth 

 century of ours, specialization has become the order of the day. 



A good example of a one-celled animal is the amoeba, which is 

 often found floating in stagnant water, and which consists of 

 nothing more than a mass of tissue, or protoplasm, as it is called, 

 and yet this apparently simple structure can move from place to 

 place, it can pick up and incorporate with its one substance par- 

 ticles of food with which it comes in contact, it can store up as 

 granules certain of these foodstuffs, and get rid of others that it 

 does not require ; it grows as a result of this incorporation, until 

 at last it splits in two and each half repeats the cycle. In other 

 words, this single cell shows all of the so-called attributes of life : 

 movement, digestion and assimilation of food, growth and repro- 

 duction. No one of these properties is necessarily confined to 

 living structures alone, for some perfectly inanimate bodies may 

 exhibit one or other of them, yet when all occur together, we 

 consider the structure. to be living. 



In the higher animals, these functions are performed by the 

 so-called systems, such as the digestive, the circulatory, the res- 

 piratory, the excretory, the motor, the nervous and the reproduc- 

 tive, each system being composed of certain organs and tissues 

 which are designed for the special purpose of carrying out some 

 particular function, or functions. One function, however, is com- 

 mon to all of the organs and tissues, namely, that of nutrition, 



