These two bodies are therefore representatives of two distinct groups or radicals present in the hemi-portion of the proteid molecule; the first belonging to the fatty acid series, the second to the aromatic group from which come such well-known bodies as indol, skatol, benzoic acid, and other substances prominent in proteid metabolism. Moreover, these two hydrolytic products of trypsin-proteolysis are formed in considerable quantity, at least in an artificial digestion. Thus, Kühne has reported the finding of 9.1 per cent. of leucin and 3.8 per cent. of tyrosin as the result of a typical digestion, and I have tried many similar experiments with like results. Further, we know from observations made by different investigators that both leucin and tyrosin may be formed in considerable quantities in trypsin-proteolysis as it occurs in the living intestine. But to this point we shall return later on. Besides leucin and tyrosin, aspartic acid and glutamic acid have long been known as decomposition-products of the vegetable proteids. Thus, both acids were discovered by Ritthausen and Kreusler[179] in the cleavage of such proteids by boiling dilute acid. Hlasiwetz and Habermann[180] likewise obtained aspartic acid in large quantity by the breaking down of animal proteids under the influence of bromine. Further, Siegfried[181] has recently obtained glutamic acid as a product of the decomposition of the phosphorus-containing proteid, reticulin, from adenoid tissue. As products of trypsin-proteolysis, Salkowski and Radziejewski[182] found aspartic acid in the digestion of blood-fibrin; and v. Knieriem[183] likewise obtained it in the digestion of gluten from wheat. Both of these acids belong to the fatty acid series, the aspartic acid being a dibasic acid, COOH. CH_{2}CH(NH_{2}). COOH, or amido-succinic acid, while glutamic acid, COOH. C_{3}H_{5}(NH_{2}). COOH, is likewise a dibasic acid, known as amido-pyrotartaric acid. [179] Verbreitung der Asparaginsäure und Glutaminsäure unter den Zersetzungs-producten der Proteinstoffe. Journal f. prakt. Chemie, Band 3, p. 314. [180] Ueber die Proteinstoffe. Liebig’s Annalen, Band 159, p. 304. [181] Ueber die chemischen Eigenschaften des Reticulirten Gewebes. Habilitationschrift. Leipzig, 1892. [182] Bildung von Asparaginsäure bei der Pancreas-Verdauung. Bericht. d. Deutsch. chem. Gesellsch., Band 7, p. 1050. [183] Asparaginsäure, ein Product der künstlichen Verdauung von Kleber durch die Pancreas-Drüse. Zeitschr. f. Biol., Band 11, p. 198.