300 THE MAINTENANCE OF THE INDIVIDUAL 



reserve in adipose tissues. Sonic animals must build up a large sui)ply 

 of fat so that they may draw upon it when their food supply is low. 

 This is particularly true of such hibernating animals as the bear that 

 emerges in the spring from a period of sleep at a time when its fat 

 supply is depleted. Fat storage in man, upon the other hand, is 

 entirely unnecessary from a physiological point of view and, due to 

 the frequency of meals, is usually quite involuntary. 



SECTION B. THE HOW AND WHY OF CIRCULATION 



Why a Transportation System? 



Within the body of nearly all of the metazoa evidence of a highly 

 specialized system of internal transportation is found. The degree of 

 development of such a system depends mostly upon the size of the 

 organism, the amount of activity it displays, the complexity of its 

 internal organization, and whether or not it is a warm blooded animal. 

 The size of the body, the speed and frequency with which the animal 

 moves are some of the factors that determine how "specialized and 

 well trained" the "handy man" about the body, i.e., the circulatory 

 system, must be. With specialization comes greater division of 

 labor, yet specialized parts such as nerve cells and muscle fibers require 

 not only nourishment but also the elimination of waste products 

 from their immediate vicinity as well as favorable conditions of 

 temperature. The solution of the problem is met in part by more or 

 less bathing all cells in lymph which serves for bringing food to the 

 cells and for the removal of wastes. In order to secure a continuous 

 food supply and to insure the adequate removal of wastes such a 

 transportation system is necessary. 



In all but the simplest organisms such a system is composed of 

 vessels containing lymph which brings its contents to locations where 

 it can eliminate the wastes, take up the energy-releasing oxygen, and 

 pick up food for the tissues. Without such a system the organism 

 cannot exist. 



Unspecialized Transportation Systems 



Unicellular animals obviously have no need for a circulatory 

 system as each individual cell is in a position to excrete its own 

 wastes and secure oxygen and food for itself through its own cell 

 membrane. Even in slightly more specialized forms, such as the 



