Chap. IX.] NUTRITION AND METABOLISM. 189 



blood and partly into the lymphatic system. We have now to 

 note that these products do not pass unaltered into the general 

 system of the circulation. For the blood which receives the 

 diffusible products has to pass through the liver before it reaches 

 the heart; and the chyle, as the milky fluid in the lacteals is 

 termed, has to pass through the lymphatic glands before it 

 passes by the thoracic duct into the blood-stream. 



The exact changes which go on in the liver have still to be 

 fully worked out. It is certainly the seat of most important 

 chemical changes, and the high temperature of the blood which 

 leaves this organ seems to indicate that katabolic changes are in 

 excess. Of these chemical changes perhaps the most important 

 is the metabolism of nitrogenous matters. From this meta- 

 bolism, effected by the vital activity of the hepatic cells, three 

 products res'ult : (1) a non-nitrogenous starchy substance, 

 readily convertible into glucose and termed glycogen; (2) the 

 slightly nitrogenous bile : and (3) the highly nitrogenous urea-, 

 or a mesostate thereof. Of these the bile, as we have already 

 seen, is poured out into the alimentary canal ; the urea passes 

 away in the blood to be eliminated by the kidneys ; the destina- 

 tion of the glycogen is still a imtter of uncertainty. By some it 

 is held to be at once converted into glucose, which, by oxida- 

 tion in the lungs, is a source of heat to the body. By others it 

 is regarded as a store of carbo-hydrate ready to be converted 

 into sugar and drawn upon by the organism when need arises. 

 By others it is regarded as a mesostate in the formation of fat. 



In any case we see that the series of metabolic changes which 

 the absorbed material has to undergo are initiated in the liver 

 before the portal blood reaches the heart. So, too, do important 

 metabolic changes take place in the chyle before it passes into 

 the blood-stream by the thoracic duct. 



The lymph may be regarded as to a large extent the overflow 

 of the blood, the plasma of which has exuded into the tissues, 

 and has been drained off by the lymphatic vessels. In the 

 region of the alimentary canal, the lymph, during digestion, 

 becomes chyle, a milky, slightly alkaline fluid, the whiteness 

 and opacity of which is due to the emulsion absorbed by the 



