206 THE VITAL PROCESSES 



ammonia, which pass into the blood and are then taken up 

 by the liver. By the action of the liver cells these are 

 converted into urea and this is turned back into the blood. 

 From the blood the urea is separated by the secreting cells 

 of the kidneys. 



Work of the Liver. The liver, already described as an 

 organ of digestion (page 152), assists in the work of excretion 

 both 1 by changing waste nitrogenous compounds into urea 

 and by removing from the blood the wastes found in the 

 bile. While the chief work of the liver is perhaps not 

 that of excretion, its functions may here, be summarized. 

 The liver is, first of all, a manufacturing organ, producing, 

 as we have seen, three distinct products bile, glycogen, 

 and urea. On account of the nature of the urea and the 

 bile, the liver is properly classed as an excretory organ; 

 but in the formation of the glycogen it plays the part of 

 a storage organ. Then, on account of the use made of the 

 bile after it is passed into the food canal, the liver is also 

 classed as a digestive organ. These different functions 

 make of the liver an organ of the first importance. 



Excretory Work of the Food Canal. The glands con- 

 nected with the food canal, other .than the liver, while 

 secreting liquids that aid in digestion, also separate waste 

 materials from the blood. These are passed into the canal, 

 whence they leave "the body with the undigested portions 

 of the food and the waste from the liver. Though the 

 nature and quantity of the materials removed by these 

 glands have not been fully determined, recent investiga- 

 tions have tended to enhance the importance attached to 

 this mode of excretion. 



The Perspiratory Glands. The perspiratory, or sweat, 

 glands are located in the skin. They belong to the type 

 of simple tubular glands and are very numerous over the 



