ACTION OF THE ENZYMES. 121 



stand in marked contrast to those of- the original protein sub- 

 stance for instance, fibrin which is insoluble and readily 

 held back by a filter or animal membrane. It is these prop- 

 erties of the products of gastric digestion which give them the 

 power of being easily taken up by the mucous membrane of 

 the alimentary tract. 



Our conceptions of the nature of the products formed when 

 proteins are digested in the presence of acid-proteinase have, 

 within the last few- years, been markedly altered, more espe- 

 cially through the investigations of ZUNZ/ PFAUNDLER, 2 LAW- 



ROW, 3 MALFATTI, 4 SALASKIN, 5 LANGSTEIN, 6 EMIL FlSCHER 7 



and ABDERHALDEN. 7 The work of all these authors indicates 

 that acid-proteinase working in the presence of an acid causes 

 a cleavage of the protein molecule into substances of a much 

 simpler chemical composition than we formerly supposed. 

 Where we once believed that proteoses and "peptones," in 

 the sense in which KUHNE used this term, constituted the 

 final products of gastric digestion, we now know that a large 

 number of substances, which were formerly looked upon as 

 produced only in pancreatic digestion, are also formed. In 

 experiments in which acid and acid-proteinase have been 

 allowed to act long enough, there have been found, besides the 

 more complex proteoses and "peptones," the following simple 

 compounds: leucin, tyrosin, alanin, phenylalanin, amino- 



1 ZUNZ: Zeitschrift f iir physiologische Chemie, 1899, XXVIII, p. 132. 



2 PFAUNDLER. Zeitschrift fur physiologische Chemie, 1900, XXX, 

 p. 90. 



3 LAWROw: Zeitschrift fiir physiologische Chemie, 1899, XXVI, p. 

 513; ibid., 1901, XXXIII, p. 312. 



* MALFATTI: Zeitschrift fur physiologische Chemie, 1900, XXXI, p. 

 43. 



5 SALASKIN: Zeitschrift fiir physiologische Chemie, 1901, XXXII, p. 

 592. 



6 LANGSTEIN: HOFMEISTER'S Beitrage zur chemischen Physiologic, 

 1901, 1, p. 507. 



7 EMIL FISCHER and ABDERHALDEN: Zeitschrift fiir physiologische 

 Chemie, 1903, XXXIX, p. 81. 



