ABSORPTION AND THE ROUTES OF FOOD. 363 



the sugar continue. That portion of the proteids taken 

 which is needed to maintain the proper composition of the 

 blood is then without change of any kind allowed to pass 

 through the liver and added to the general blood stream. 



The thing in the albumens which makes them impos- 

 sible to be stored is the nitrogen they contain. This nitro- 

 gen, as pointed out, is burned into the substance called 

 "urea," is then allowed to drop back into the blood stream, 

 and from this blood stream it is eliminated by the kidneys. 

 The liver is therefore the seat of the urea formation. But 

 it may be well at this place to point out that there is a 

 second source from which the liver makes its urea. This 

 second source is from the burned up tissues. Various pro- 

 ducts of tissue disintegration (and tissues are largely al- 

 buminous) reach the liver, and by the liver are burned into 

 urea and then sent to the kidneys, as in the first case. The 

 source of the urea is therefore a double one ; one directly 

 from the burning of the excessive albumens in the liver, 

 the second from the burning of tissue wastes sent to the 

 liver from all the organs of the body. The liver is, there- 

 fore, not only a storehouse, it is to some extent also a cre- 

 matory for the nitrogenous wastes. 



The final state of things is then as follows : 

 First. Definite amounts of the albumens have been 

 allowed to pass through the liver and circulate in the 

 blood. 



Second. Quantities of dextrose or grape sugar are from 

 time to time doled out from the reserve supply of glycogen 

 in the liver and dropped into the blood stream. 



Third. Into this blood stream are carried all the fats 

 absorbed by the lacteals. 



Fourth. In this blood, too, are the various mineral 

 salts and the water taken in the foods. 



The question which now presents itself is, in what 

 manner are these nutritive factors of the blood used by the 



