THE TJEINB 371 



blood of the glomeruli. It was once supposed that the 

 glomerulus was simply a filter, and that the waste and salts 

 passed tlirough by filtration and osmosis. Present the- 

 ories tend to prove that the cells of the capsule actually 

 secrete this water and salt material first received from the 

 blood. The cells of the upper parts of the tubules secrete 

 the organic wastes (urea, etc.) which they obtain from the 

 capillaries surrounding these parts and which is dissolved 

 in the water to make the urine. The water, salts, and 

 organic wastes accumulate slowly, and drop by drop they 

 pass through the length of the tubes into the calices and 

 pelvis. Here they enter the ureter, and the combined 

 liquid is stored in the bladder pending removal. The blad- 

 der is covered with a layer of muscle and peritoneum, and 

 lined with mucous membrane, and the mouth of the urethra 

 is closed by a sphincter muscle which opens from time to 

 time to permit the escape of the urine. 



The urine. — -Analysis of the urine shows it to consist 

 of a yellowish liquid slightly heavier than water and con- 

 sisting of about 96% water and 4% solids. The principal 

 solid constituent is a nitrogen waste called urea, and the 

 most important excretory action of the kidneys is their 

 power to remove the nitrogen wastes of the body. 



In the metabolism of the tissue cells the nutrients are 

 broken up into water, salts, carbon dioxide, and nitrogen 

 wastes. We have already seen that the water as sweat, 

 is largely excreted by the skin, and the carbon dioxide by 

 the lungs. The kidneys, therefore, are the great nitro- 

 genous waste disposers. The total amount of water, salts, 

 and nitrogenous wastes given off by the kidneys in twenty- 

 four hours varies from forty to sixty fluid ounces or one 

 thousand five hundred grams. Of this, water forms 96%, 



