So far there is little possibility of dispute; but now we reach the realm of theory, and there is much contention as to the future course of events. Kuhne's view of protein digestion and absorption was that hemi-peptones and anti-peptones were formed, the former being absorbed, the latter giving rise to leucin, tyrosin, and other amino-acids not capable of absorption, being therefore wasted.

But we know that there are many kinds of proteins. Osborne has discovered quite twenty in wheat alone; and we know that they are split up by hydrolysis into at least twenty different amino-acids, amongst which are leucin, tyrosin, glycocoll, cystin, arginin, tryptophane, and histidine. As we have seen, this decomposition occurs in successive stages from complex to simple, and it would be interesting, if it would not take up too much time, to study the probable method of formation of each.

Probably the simplest example of an amino-acid is glycocoll or glycine, which is practically a fatty acid, viz., acetic acid, CH3COOH, with one of the three hydrogen atoms in the CH3 group replaced by the amino-group NH2, forming ammo-acetic acid or glycine, CH2NH2COOH.

Alanine is amino-propionic acid, propionic acid being the next number of the fatty acid series. An extremely complex specimen is tryptophane, a near relation of the last mentioned. It is indoi-amino-propionic acid, and is the parent substance of two foul-smelling products of protein decomposition called indol and skatol. Amino-acids are divided into two classes according as they contain one or two amino groups, hence called mono-amino-acids and di-amino-acids respectively, and the former is still further subdivided into three groups.

Had this information been at the command of the earlier physiologists, fewer mistakes would have been committed in connection with the metabolism of proteins. Even now it is a moot point what happens to the spilt-up products of protein digestion. Kuhne's view, which has been just mentioned, is obviously incorrect, because it is known that peptones injected into the blood act as reducers of blood pressure, producing collapse from toxaemia and subsequent peptonuria. To combat this difficulty it was assumed that intestinal products of digestion are dehydrated by the epithelial cells of the bowel-wall into blood-plasma proteins, and on this was constructed the "circulating protein" theory of Voit, which stated that the serum albumin and globulin were the ultimate forms of protein nourishment for the living cells. Hofmeister's view was that peptone was absorbed by intestinal lymphocytes, and in this bound-up condition traversed the circulatory system without filtering through the kidneys - an obvious parallel to Pavy's theory.

It is now taught by some that the amino-acids are absorbed by the intestinal epithelium, and there reconstructed into the proteins of the blood, these again being disintegrated by the cells of the tissue, and from these ultimate products the specific proteins of the tissues are formed. Others, again, maintain that there is no immediate re-synthesis, but that after absorption the products are presented in turn to the different tissues, which are thus permitted to make a selection according to their requirements.

Future chemists will doubtless be able to trace the complex protein molecule right down to its simplest ultimate fragments, and demonstrate the manner in which they are utilised in the construction of each individual tissue, but despite the brilliant results of Emil Fischer's labours we have made little progress towards the elucidation of this problem.