HUMAN STRUCTURE AND LIFE- ACTIVITIES 509 



it to the cells of the entire body. (2) The lungs are adapted 

 to supplying the blood with oxygen, which is then distrib- 

 uted to the cells. (3) The excretory organs (chiefly lungs 

 and kidneys) are adapted to remove from the blood the 

 various excretions found in the cells. (4) The circulating 

 liquids (blood and lymph) serve the functions of food-supply, 

 oxygen-supply, and excretion, in that food and oxygen 

 must be distributed to the cells and excretions taken away 

 to the excretory organs. 



These four great functions, involving the organs of digestion, 

 respiration, excretion, and circulation, serve the cells. We 

 have noted ( 268) that a one-celled organism does not need 

 such a complicated mechanism, for with respect to food, 

 oxygen, and excretions it can deal directly with the external 

 world. The vast number of cells in a higher animal has 

 made necessary the complicated organs of digestion, respira- 

 tion, excretion, and circulation for serving the cells. 



NERVOUS ACTIVITY 



437. Need of Coordination. The functions which serve 

 the cells named in the preceding paragraph must work to- 

 gether or in harmony. For instance, if muscles are working 

 faster, there is need of more food and oxygen. This requires 

 more rapid circulation, greater oxygen-supply, and increased 

 digestion in order to supply and transport the necessary 

 food and oxygen. And faster work results in more excretions 

 and consequently greater activity of the circulatory and ex- 

 cretory organs in removing them. Thus increased activity 

 of certain organs may demand a corresponding increase in 

 work of many others ; and there must be coordinated activity. 



The function of coordination is part of the work of the 

 nervous system. This is parallel with the case of a frog's 

 functions ( 54) ; but also the human nervous system has a 

 vast amount of other work arising from mental activities. 



