DIGESTION 



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The end-products of the digestion of proteins are the 

 amino-acids, which contain nitrogen in the form of amino 

 or NHo groups. In the Hver these are spHt off to form 

 ammonia, which combines with carbon dioxide to form 

 ammonium carbonate. By removal of water, probably 

 in the liver cells, this compound is converted into urea, 

 which is excreted from the body by the kidneys. The 

 nitrogen-free residues of the amino-acids have either a 

 carbohydrate or a fatty character, and yield energy to the 

 body by being oxidised. 



Although the fats absorbed from the intestine do not 

 pass through the portal system directly, the liver plays a 

 part in their metabolism. Some of the links of the long 

 carbon-atom chains of the fatty acids are weakened, so 

 that their subsequent oxidation is facilitated ; and fatty 

 acids are also combined with phosphoric acid to form 

 lecithin, which is more easily transported about the body 

 than the true fats. There is no special organ for the 

 regulation of the amount of fat ; the drops pass through 

 the walls of the capillaries and are stored in connective 

 tissue cells. The liver has many other functions, for 

 example, in the preparation of the bile, which contains 

 both valuable adjuvants to digestion and useless waste 

 products ; it is, in short, the most important chemical 

 clearing-house of the body. 



Many Invertebrates, such as Molluscs and Crustacea, 

 possess a large digestive gland called the " liver " or 

 hepatopancreas. It combines the function of the verte- 

 brate pancreas, in secreting digestive enzymes, with that 

 of the liver in storing absorbed food and probably in 

 carrying out various chemical reactions and setting free 

 waste products ; and very often there takes place 

 intracellular digestion and assimilation of particles 

 of food brought up from the alimentary canal proper 

 — a function unknown in Vertebrates. The relative 

 importance of these activities varies from group to 

 group. 



Respiration. — There is another most important material 

 to be noticed, namely, the oxygen which is absorbed from 

 the air by the lungs. We may picture a lung as an elastic 

 sponge-work of air chambers, with innumerable blood 



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