THE DIGESTIVE SYSTEM 105 



body. The amino-acids pass through the liver unchanged and go 

 to the muscles and other tissues of the body where they appear to 

 be synthetized into such body-proteins as are found in muscle and 

 ligamentous tissue, or otherwise made available as a source of 

 energy and growth for the body. At one time it was believed 

 that the proteins were broken down only so far as proteoses and 

 peptones, in which forms they were absorbed. Now we know 

 that there is a further cleavage, for not only is it impossible to 

 find even traces of these substances in the blood of animals fed 

 on a very high protein diet, but by injecting purified proteose 

 and peptone into the blood-stream of experimental animals death 

 occurs at once, due to the toxicity of these substances. The 

 theory at present most generally accepted is that protein sub- 

 stances are absorbed as amino-acids; the final conversion being 

 the result of the action of the enzyme erepsin. 



Glucose is the form in which carbohydrate materials are ab- 

 sorbed by the blood. When glucose reaches the liver it is believed 

 that a part immediately passes through this organ unchanged. 

 The balance is converted into glycogen and stored for future use. 

 The fact that the muscles also have the ability to convert glucose 

 and store it as glycogen should be recalled. More complex forms 

 of sugar, for example maltose, cannot be made use of by the 

 organism without being inverted. This is shown by injecting 

 maltose subcutaneously, in which event it is excreted unchanged 

 in the urine. 



The issue of sugar from the liver is surprisingly constant no 

 matter how much is being supplied to it by the blood collected 

 from the intestines. The means by which the output is controlled 

 is not fully known. It is surmised that an internal secretion or 

 hormone produced by the pancreas and carried to the liver by the 

 blood is the influential factor, for when the pancreas is removed 

 experimentally the urine is soon found to contain sugar. While 

 glucose is ordinarily absorbed in very large amounts, complete 

 removal of carbohydrates from the ration does not result in death 

 as is the case in removal of proteins. 



Water is absorbed in part by the blood-vessels, the process 

 being very rapid if there is a deficiency in the tissues. There is 

 difference of opinion as to the mechanism of water absorption 

 from the small intestine. Some authorities consider it is brought 

 about by diffusion, osmosis, filtration, and other physical proc- 



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