Page:Popular Science Monthly Volume 72.djvu/393

 or less diffusible modifications adapted for absorption into the circulating blood. The various forms of vegetable and animal proteins supposedly underwent transmutation without much chemical change, into closely allied substances which by simple osmosis or diffusion could enter the circulation and thus be distributed throughout the body. We see in these views a striking example of how preconceived ideas stand in the way of progress. Advance of knowledge is frequently held back by our proneness to interpret observations or data in harmony with our conception of what they should signify. The old-time physiologists knew full well that the protein of the blood and tissues was made good by the protein of the food, and digestion as they understood it was adequate for the purpose, viz., to transform the protein into a soluble and diffusible form. Anything beyond this was not only unnecessary, but uneconomical and wasteful. I well remember an experience of my own twenty-five years ago in Germany when I was at work on the so-called primary and secondary proteoses formed in gastric digestion, substances at that time just discovered as products of digestive action; how an eminent physiologist from Dorpat happened in the laboratory one day, and after looking at some of the products he turned to Kühne, who was standing near-by and remarked, "It is certainly interesting to see what changes pepsin is capable of producing, but of course they have little bearing on the processes that take place in the stomach and intestine, where naturally the sole aim is to fit protein for absorption."

Another eminent physiologist, whose name has long been known to every student of the science, remarked once in my hearing that the acid-albumin stage, the first product formed by pepsin-acid, was as far as gastric digestion need extend, since this substance was easily absorbed and it was a useless waste of energy for protein to undergo conversion into primary and secondary proteoses and peptone. I recall also what controversies arose when it was established that artificial pancreatic juice could break down protein into leucine and tyrosine; those two crystalline amino-acids now so well known as decomposition products of most proteins. When the fact was established beyond a shadow of doubt, physiologists were still disinclined to believe that any appreciable amount of these relatively simple substances could be formed in ordinary digestion, because such a view was so strongly opposed to the general purpose of protein digestion as then held. We were not inclined to follow the path which experiment was opening up simply because our eyes were blinded by preconceived ideas. I recall an early experiment made by Kühne and myself in the Heidelberg laboratory where a dog was fed a large amount of meat and then after a suitable time chloroformed, the small intestine ligatured and the contents analyzed. We found a gram or more of leucine and tyrosine, which we weighed and identified, thus proving to our satisfaction at least that these two