331 



foundation to this theory of proteinogenesis. On feeding dogs 

 with starch and cane sugar, together with the soluble nitrogenous 

 substances of the pancreas digested until the biuret reaction 

 completely disappears he found that the animals were not only 

 capable of maintaining their nitrogenous equilibrium, but were 

 also able to store up a considerable amount of nitrogenous 

 substance. 



This process of the reconstruction of proteins by the combin- 

 ing together of amino-acids with loss of water (polymerisation) is 

 made more intelligible by the recent work of Emil Fischer, 

 who (as stated in Vol. I. p. 28) succeeded by the synthesis of 

 different amino-acids in obtaining much more complex compounds, 

 which he termed polypeptides. In some of their properties these 

 resemble the proteins. 



XIV. To complete this chapter we must draw attention to 

 certain important experimental data, which show that the epi- 

 thelial cells that line the gastro-intestinal canal have (like the 

 hepatic cells), besides the secretory function we have discussed, a 

 protective function. By this they are able to dimmish or inhibit 

 the effects of toxic substances, whether introduced from without 

 or formed within the body, especially in the intestinal canal, 

 owing to the processes of digestion and putrefaction which take 

 place there. This protective function is more particularly built 

 up on that capacity for physiological selection by which the 

 epithelia of the intestine, while they absorb certain substances (in 

 defiance of the laws of osmosis) that diffuse with difficulty, do not, 

 on the other hand, permit others to pass which are more diffusible. 

 This selective capacity, of course, has a limit, otherwise intoxica- 

 tion by gastro-intestinal paths could not take place, which is an 

 obvious absurdity. It is certain that many poisons of the category 

 of alkaloids produce a greater toxic effect when injected under the 

 skin than when administered by the mouth. It is often found 

 that the lowest lethal dose, as given hypodermically, is innocuous 

 or far less harmful when swallowed. M. Schiff (1861) and 

 F. Lussana (1864) were the first to demonstrate this point. 



The phenomenon depends not merely upon the slow rate at 

 which alkaloids and other toxic substances are absorbed by the 

 intestinal epithelium, but also on the fact that after absorption 

 they pass by the roots of the portal system to the liver, where 

 they are arrested by the hepatic cells, which store up the alkaloids 

 in their cytoplasm, partly destroying them, partly restoring them 

 to the intestine with the bile, and partly discharging them by the 

 hepatic veins to be eliminated by the kidneys. Schiff actually 

 found the fatal dose of narcotic poisons to be much lower when 

 they were introduced hypodermically, than when they were 

 injected directly by the portal vein. He further found that a 

 frog, in which the vessels to the liver had been tied, died after the 



