THE DIGESTIVE SYSTEM 85 



tion; these are: (1) the small veins that run together to form the 

 portal vein of the liver; (2) the lacteals, which convey by way of the 

 mesenteries through groups of lymphatic glands to the receptacu- 

 lum chyli and then into the thoracic duct (Fig. 24) . 



The portal vein carries all the blood from the stomach, spleen, 

 and intestines to the liver, which is placed between the digestive 

 and circulatory systems. Through it must pass all the products 

 of protein and carbohydrate digestion. These include amino-acids, 

 carbohydrates, ammonia, and toxic materials. 



Amino-acids comprise a number of organic substances, among 

 which are aspartic acid, arginin, cystin, histidin, leucin, lysin, 

 phenylamin, prolin, tryptophan, and tyrosin. When any one of 

 these is lacking in the ration, starvation will ensue. They repre- 

 sent the last stage in protein digestion; further cleavage would 

 probably render the nutrients unsuitable for use by the 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 muscle and ligamentous 

 tissue, or otherwise made available as a source of energy and heat. 

 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 experimen- 

 tal animals death ensues at once, due to their toxicity. The theory 

 at present most generally accepted is that protein substances are 

 absorbed as amino-acids, the final conversion being the result of the 

 action of the enzyme erepsin. 



Carbohydrates are found in the circulating blood, liver, and 

 muscles. They are absorbed as glucose. The animal can subsist 

 if no carbohydrates are in the food. 



Ammonia originates, according to Folin, almost entirely in the 

 large intestine as a result of putrefaction of the residue of diges- 

 tion. It is largely disposed of in the liver by combining to form 

 urea, the chief waste substance excreted in the urine. 



Toxic materials are absorbed in considerable amounts from the 

 intestines. Upon reaching the liver they are destroyed or ren- 

 dered harmless. This important function of the liver was demon- 

 strated nicely by Schiff, who injected a dose of nicotin through a 



