50 ELEMENTS OF BIOLOGY 



A group of organs which have a common major purpose is con- 

 sidered an ORGAN SYSTEM, as all the organs concerned in the diges- 

 tion of foods in Man form the digestive system. In a metazoon body 

 the several organ systems are interdependent. For our purposes here 

 we may consider an individual to consist of the essential inter- 

 dependent organ systems that enable a body to perform the func- 

 tions of life. 



Cooperation. The relation of cells, tissues, organs, and organ 

 systems to each other and to the individual as a whole may be com- 

 pared with the relation of private soldiers and the various military 

 services to the function and mission of a field army. An army func- 

 tions because of the sum of the eflforts of the individual soldiers, but 

 the duty of any one soldier is only an exceedingly minor part of the 

 whole function of the army and may have no apparent relation to 

 the mission of the larger force. In order to carry on as a functional 

 unity, various types of soldiers are organized into groups of spe- 

 cialists; for example, the signal service, the service of supply, and so 

 on. So also in the multicellular organism, the functioning of the 

 individual is the sum of the functions of the individual cells, but 

 the function of each cell is not identical with that of the whole 

 organism. The various tissues are to be compared with the specialist 

 branches of an army organization, it being the sum total of the 

 diverse functions of the several tissues and organs that brings about 

 the characteristics of the individual. The analogy may be carried 

 still further. The effectiveness of an army depends on the physical 

 condition, abilities, training, and general morale of all the units of 

 the service. The characteristics of an animal are conditioned upon 

 the efficiency and activities of the cells that make up its several 

 special tissues, 



Suggested Readings 



Ponder, E.: Essentials of General Physiology. Chapters 3 and 12. Long- 

 mans, Green and Company, 1929. 



