694 PHYSIOLOGY OF DIGESTION AND SECRETION. 



Although several members of this group have been isolated and given 

 separate names, so much doubt prevails as to the chemical individ- 

 uality of these substances that it is best perhaps to regard them as a 

 group of compounds which under the continued influence of the 

 pepsin undergo still further hydrolysis with the formation of secon- 

 dary proteoses or deutero-albumoses. As compared with the primary 

 proteoses, the secondary ones are distinguished by a greater solu- 

 bility ; they require a stronger saturation with neutral salts to precipi- 

 tate them. (See appendix.) The secondary proteids undergo still 

 further hydrolysis, with the production of peptone, or perhaps it 

 would be better to say peptones. The peptones show still greater 

 solubility, and, in fact, peptone, in Kiihne's sense, is that compound 

 or group of compounds formed in peptic digestion which, while still 

 showing proteid reactions (biuret reaction), is not coagulated by 

 heat nor precipitated when its solutions are completely saturated 

 with ammonium sulphate. According to the schema and descrip- 

 tion given above, the several stages in peptic digestion are repre- 

 sented as following in sequence. It should be stated, however, 

 that many authors consider that even in the beginning of the 

 digestion the proteid molecule may be split into several complexes, 

 and that some of the end-products may be formed in the very 

 beginning of the action. All that we can state very positively 

 is that the proteid molecules undergo a series of hydrolytic cleav- 

 ages, the end-result of which is that in place of the originally very 

 large molecule with a weight of 5000 to 7000 there is obtained a 

 number of much smaller and much more soluble molecules whose 

 molecular weights are perhaps only 250 to 400 or less. 



It was formerly believed that pepsin was not able to split the complex 

 proteid molecule into compounds of a simpler structure than the peptone. 

 But a number of recent authors have stated that if time enough is given 

 the breaking up of the proteid molecule may be as complete as after the action 

 of trypsin, or after hydrolysis by acids (see Proteids in appendix). That 

 is, along with the peptone or in place of it are found certain simpler bodies 

 which no longer give the biuret reaction, but are precipitable by phospho- 

 tungstic acid and for which Hofmeister proposes the general name of pep- 

 toids. They would correspond, also, apparently, to the group of compounds 

 designated by Fischer as peptids or polypeptids. In addition, many of the 

 amido-acids and nitrogenous bases which constitute the final end-products 

 of the breaking up of the proteid molecule may be found.* 



In judging the digestive action of any given specimen of natural 

 or artificial gastric juice it is customary to measure the rapidity 

 with which an insoluble proteid is converted into a soluble form. 

 The method most commonly employed is that devised in Pawlow's 

 laboratory by Mett. The Mett test is made by sucking white of egg 

 into a thin-walled glass tube having an internal diameter of 1 to 2 

 rnms. The egg-albumin is coagulated in the tube by immersing it for 

 * See Hofmeister, " Ergebnisse der Physiologic," vol. i, part i, 796, 1902. 



