TISSUES, ORGANS, AND PHYSIOLOGICAL SYSTEMS 37 



So, too, we find a respiratory system consisting primarily of two 

 hollow organs, the lungs, which lie in the chest and communicate 

 by the windpipe with the back of the throat, from which air enters 

 them. But to complete the respiratory apparatus are many other 

 organs, bones, muscles, nerves, and nerve-centers, which work to- 

 gether to renew the air in the lungs from time to time; and the 

 act of breathing is the final result of the activity of the whole 

 apparatus. 



The Relation of Man to His Environment. From infancy the 

 human organism is confronted with the task of maintaining itself 

 alive. To this end all the bodily functions bend themselves. The 

 maintenance of life in man, as in all animals, presents two distinct 

 problems: first, to obtain the necessary food; and second, to cope 

 successfully with the innumerable perils with which the organism 

 is continually confronted. Failure in either of these endeavors 

 means failure in maintaining life itself. 



The labor of obtaining food and the struggle to escape harm take 

 place in the midst of a world filled with creatures engaged in the 

 same labor and the same struggle. Indeed it is the very prevalence 

 of living beings that makes the securing of food labor, and the 

 avoidance of harm a struggle. All the living beings that belong to 

 the animal kingdom are in a more or less continuous state of 

 activity. Each individual, therefore, finds himself surrounded by 

 a continually shifting world of other beings. Nor is inanimate 

 Nature stationary; winds and rains, heat and cold, come and go. 

 To such a constantly changing environment the organism must 

 adapt itself. 



In the complex of systems which together make up the body it 

 is possible to distinguish between those whose immediate function 

 is to maintain the necessary adaptation of the organism to its 

 environment and those which function only indirectly to that end 

 by keeping the body itself in good working order and each part 

 well supplied with the energy yielding materials without which 

 activity is impossible. In making such a distinction, however, it 

 must be borne in mind that all the bodily functions work together 

 for the good of the whole body so that no hard and fast line can be 

 drawn between the two classes of systems. It will be convenient 

 to consider first the systems which are particularly concerned in 

 adapting the body to its environment. 



