MAN 303 



dioxide and still be fit for breathing, but a greater amount 

 is injurious. 



The excretions formed as a result of metabolism are 

 chiefly eliminated through the kidneys, lungs, and skin. 

 The kidneys discharge the urea, formed as one of the end 

 products when proteins break down. Though urea is 

 excreted through the kidneys, it is formed for the most 

 part in the liver and transferred in the blood. Water passes 

 out through all three of the excretory channels; carbon 

 dioxide is eliminated largely through the lungs, but some 

 goes through the skin. All the excretory organs are con- 

 trolled and coordinated by the nervous system. For 

 example, in cool weather there is less water passing out 

 through the skin and more through the kidneys. 



The activities of the human body are more or less rhyth- 

 mical. The day is usually a time of activity and the body 

 is used up to some extent in doing work; night is a period 

 of rest and losses are made good during sleep. Sleep is a 

 curious phenomenon sense organs which are at other 

 times quick to receive stimuli become inactive and all 

 extracorporal activities cease. There have been a number 

 of theories to explain sleep and the three following may be 

 mentioned: (1) Sleep may be caused by the using up of sub- 

 stances necessary for nervous activity; (2) it may be due 

 to the accumulation of waste products which cause the 

 enlargement of small blood-vessels throughout the body 

 with a resulting scarcity of blood in the brain (though such 

 changes do occur, they probably do not cause sleep); (3) 

 sleep may be due to the contraction of nerve branches so 

 that those of different cells are no longer in contact thus 

 the paths are broken along which nervous impulses travel 

 when the body is awake. Whatever its cause, sleep is 

 necessary for all warm-blooded animals, and the human 

 machine is most efficient when periods of rest and activity 

 alternate with reasonable regularity. 



Self-protection. The instinct for self-protection is as 

 strong in man as in other animals and the body has many 



