CHAPTER 13 

 INTEGRATION OF ACTIVITIES 



When many different operations are performed by the same machine, 

 it is essential that they bear some definite relation to one another. Living 

 organisms are subject to the same necessity. Their processes must dove- 

 tail into one another. When unusual exertion increases consumption of 

 energy and output of carbon dioxide, it would be disastrous were the 

 circulation not speeded up to provide oxygen and remove wastes. When 

 the circulation is accelerated, it would be inefficient not to hasten the 

 breathing movements to introduce more oxygen. In the digestive system 

 it would be wasteful to have saliva, bile, and other digestive fluids 

 secreted all the time, yet they must be produced when foods require 

 digestion. If in warm-blooded animals the temperature increases above 

 the most favorable point,* it is important that the sweat glands of the skin 

 or the breathing movement act to stop the rise. Even so simple an act as 

 walking involves so many muscles that cooperation among the several 

 units is necessary. The various organs cannot simply be wound up and, 

 clocklike, run at the same speed, thereby ensuring proper timing, for 

 many activities are carried on in response to external conditions and these 

 change at irregular intervals. 



Some means of coordination is necessary. Animals in general have 

 contrived two devices — one nervous, the other chemical — to serve this 

 end. The former has assumed the larger burden, but both are essential. 

 While it has been necessary, in describing the action of the heart, the 

 respiratory movements, and the production of digestive fluids, to refer 

 to the controls which keep these processes in tune with the rest of the 

 organism and with the environment, it is desirable now to examine the 

 mechanisms of control more specifically. 



Rise of the Nervous System. — The advantage or necessity of a 

 nervous system is attested by its very general presence in widely different 

 animals. Only a few groups are without it. It is made up of specialized 

 types of cells, whose arrangement in the body exhibits an increasing 

 complexity as other anatomical features become more complicated. 

 Animals which have simple systems of other kinds have, in general, 

 simple nervous systems. 



The simplest form of nervous system is that of Hydra. The cells 

 which are specialized for conduction in this animal ha\e long, slender 



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