CHAPTER XXVI 

 EXCRETION AND SEWAGE 



435. Getting rid of oxidized and waste substances is 

 excretion. All oxidations in the body produce carbonic 

 acid gas and water. In addition, the oxidation of albu- 

 min produces a substance called urea, which ' contains 

 the nitrogen of the albumin. These substances together 

 with the minerals or ashes left from the burned cells 

 niast continuously be excreted by the lungs, liver, intes- 

 tine, skin, and kidneys. These organs also excrete poisons 

 which are produced by disease germs. 



436. Difference between a secretion and an excretion. 

 In a general way, anything separated from the blood by 

 glands is a secretion. But the term strictly is applied only 

 to those substances which, like saliva and gastric juice, 

 are of use to the body. Substances which, like carbonic 

 acid gas and urea, are only waste and harmful products, 

 are true excretions. 



437. Sweat glands. Numerous coiled tubes lined with 

 epithelium project into the skin over nearly its whole sur- 

 face. Each tube is a sweat gland, whose epithelium is 

 continually secreting the sweat, or perspiration. They are 

 very numerous on the forehead, chest, palms of the hands, 

 and soles of the feet. Only a few are found in the upper 

 part of the back. 



438. The perspiration. The perspiration is over 99 per 

 cent water. It contains a small amount of urea and min- 

 eral substances. Ordinarily it evaporates so fast that its 



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