CHAPTER VIII 



THE FUNCTIONS OF THE BODY 



1. Master activities and subsidiary activities 2. Functions of the 



nervous system 3. Muscular activity 4. Nutritive functions 



5. Functions of the liver 6. Respiration 7. Excretion 



8. Organs of internal secretion 9. Functions of the blood 



10. Modern conception of Protoplasm. 



1. Master Activities and Subsidiary Activities. -- Living 

 means activity activity swayed in great part by 

 " hunger ' and " love," if we use these words in their 

 widest sense. Animals, as we have seen, busy them- 

 selves (in a delightful variety of ways) in finding food, 

 making themselves comfortable, mastering their environ- 

 ment, avoiding their enemies, wooing mates, constructing 

 nests and shelters, and tending the young. But all this 

 depends on changes within the body an animal is a 

 dynamic system for there is no movement without 

 (1) the contracting of contractile substance (usually in 

 muscle-fibres), and beyond the Unicellulars there is little 

 behaviour that does not involve (2) the activity of ner- 

 vous substance, which includes, along with " irritability," 

 all the forms of mental activity. In short, there are faro 

 master activities, as Sir Michael Foster put it, in the 

 anirnal body, contractility and irritability those of 

 muscular and those of nervous tissues. To these, the 

 other everyday activities of (3) nutrition, (4) respira- 

 tion, and (5) excretion are subsidiary or sustentative. 

 As to nutrition, the energy expended in doing work has 

 to be made good by taking in potential energy (like fuel) 

 in the form of food, and this has to be made physically 

 and chemically available (digested) before it can be in- 



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