Chapter VI 



THE STRUCTURE AND FUNCTIONS OF MULTICELLULAR 



ANIMALS 



II. Correlating Systems 



Correlating Systems. The body of a multicellular animal is 

 unified and correlated so that it behaves as an individual and not 

 as an association of independent tissues. This correlation is provided 

 for by two distinctly different types of mechanisms. One transports 

 materials to all regions so that all receive, or are in contact with the 

 same sort and quantity of food, of oxygen, and of the products of 

 metabolism. The other system is one of conduction of impulses or 

 excitations rather than of transportation of materials. The former is 

 one of the functions of the circulation, the latter primarily of the 

 nervous system, although tissues other than nervous have important 

 transmissive correlative functions. 



Role of Body Fluids as Correlating Agents. For con- 

 venience we may consider the role played by the circulatory fluids 

 as correlating agents under two headings: (i) Passively, the circu- 

 lation serves to regulate and correlate the various tissues and organs 

 by reason of the fact that it exposes each to the same constituents. 

 For example, in Man the oxygen content of arterial blood is much 

 the same in all regions, so that all tissues have the same quantity 

 available; any differences in their rates of oxygen utilization are 

 due to specific tissue differences in oxygen requirement. In warm- 

 blooded animals the temperature of the blood is fairly uniform in 



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