WASTE.] PHYSIOLOGY. 125 



We have also already seen how the blood secretes 

 sweat into the sweat-glands, and so on to the skin. 

 \ Perspiration is little more than water with a little salt 



in it. The skin, therefore, helps to purify the blood 

 through the sweat-glands, by getting rid of water with 

 a little salt. You must remember that a great deal of 

 water passes away from your skin without your know- 

 ing it. Instead of settling on the skin in drops of 

 sweat, it passes off at once as vapour or steam. Some 

 carbonic acid also makes its way from the blood 

 through the skin. 



^ 54. It only remains for us to inquire, In what way 

 does iX\t blood get rid of the ammonia and the rest 

 of the saline matters that do not pass through the 

 skin ? 



These are secreted from the blood by the kidney, 

 dissolved in a large quantity of water in the form of 

 • urine. 



What is the kidney? You will learn more about 

 this organ by and by. Meanwhile it will for our 

 present purpose be sufficient to say that a kidney is a 

 bundle of long tubular glands, not so very unlike 

 sweat-glands, all bound together into the rounded mass 

 whose appearance is familiar to you. Into these 

 glands the blood secretes urine just as it 

 secretes sweat into the sweat-glands. The 

 glands themselves unite into a common tube or duct 

 which carries the urine into the receptacle called the 

 urinary bladder, from whence it is cast put when 

 required. 



What is urine ? Urine is in reality water holding 



^ in solution several salts, and in particular containing 



a quantity of ammonia. The ammonia in urine is 



. jLV^M ** V.l.«A ^ 



