Hygiene of Digestion. 199 



Action of Diseased Kidneys. The kidney takes, during 

 health, only the waste matters, leaving the valuable nour- 

 ishing material. But, in disease, the kidneys may take out 

 some of the most valuable nourishing material. Suppose 

 that in a mill, a workman, whose business is to shovel out 

 wastes, becomes crazy, and shovels wheat or flour out of 

 the mill into the stream below. The diseased kidney may 

 be said to have become crazy, and in the disease called 

 "diabetes" throws out sugar, and in " albuminuria " ex- 

 cretes albumen. 



Blood Streams like Water Pipes and Sewer Combined. 



It is as though the water supply of a city house was taken 

 from the sewer ; each organ needing a supply of building 

 material acts like a filter, taking from the blood what it 

 needs, paying no attention to the impurities present, and 

 the organs of excretion select the impurities, allowing the 

 useful substances to pass on to the places where they are 

 needed. Figs. 85, 86, and 87 show what the blood stream 

 gives to each of the organs of the body and what it takes 

 from them to throw out as waste matter. 



How the Body Changes. The body is continually 

 changing, new material from the digested food taking the 

 place of the worn-out tissues. It is a common saying that 

 the body changes once in seven years. But while the 

 more active tissues, such as muscle, must change many 

 times in a year, we know that the teeth do not grow after 

 they are once formed. 



The Body like an Eddy. The changes in the body have 

 been compared to an eddy in a stream. The form of the 

 eddy remains the same, while particles of water are enter- 

 ing on one side and leaving on the other. In a short time 



