This latter suggestion is worthy of a moment’s further consideration. As is well known, every species of animal has its own particular type of proteid, adapted to its particular needs. The proteids of one species directly injected into the blood of another species are incapable of serving as nutriment to the body, and frequently act as poisons. Man in his wide choice of food consumes a great variety of proteids, all different in some degree from the proteids of his own tissues. Is it not possible, therefore, that it is the true function of pancreatic and intestinal digestion to break down the different proteids of the food completely into simple fragments, so that the body can reconstruct after its own particular pattern the proteids essential for its nourishment? Or, we can follow the suggestion contained in the work of Abderhalden,[10] who finds that in the long continued digestion of various proteids by pancreatic juice there results in addition to the amino-acids a very resistant residue, non-proteid in nature, which is termed polypeptid. In other words, Abderhalden believes that pepsin, trypsin, and erepsin are not capable of bringing about a _complete_ breaking down of proteids into amino-acids, but that there always remains a nucleus of the proteid not strictly proteid in nature, though related thereto,--polypeptid,--which may serve as a starting-point for the synthesis or construction of new proteid molecules, the various amino-acids being employed to finish out the structure and give the particular character desired. This view, however, is rendered somewhat untenable by the more recent experiments of Cohnheim,[11] who claims that proteids can be _completely_ broken down by pepsin, trypsin, and erepsin, and consequently polypeptids would hardly be available for the synthesis of proteids. Moreover, Bergell and Lewin[12] have ascertained that there is present in the liver an enzyme or ferment which has the power of digesting or breaking down certain dipeptids and polypeptids into amino-acids. Hence, it follows that if any polypeptids are absorbed from the intestine, they would naturally be carried to the liver, where further cleavage into fragments suitable for synthetical processes might occur. In any event, there is good ground for the belief that the more or less complete disruption of the proteid molecule into small fragments renders possible a synthetical construction of new proteid to meet the demands of the organism; a fact of great importance in our conception of the possibilities connected with this phase of proteid nutrition. [10] Emil Abderhalden: Abbau und Aufbau der Eiweisskörper im thierischen Organismus. Zeitschr. f. physiologische Chemie, Band 44, p. 27. [11] Otto Cohnheim: Zur Spaltung des Nahrungseiweisses im Darm. Zeitschrift f. physiologische Chemie, Band 49, p. 64. [12] Bergell and Lewin: Zeitschrift für experimentelle Pathologie und Therapie, Band 3, p. 425.