CHEMISTRY OF DIGESTION AND NUTRITION. 295 



separation of the peptones from the proteoses. The peptones, indeed, niav be 

 defined as being the products of proteolytic digestion which are n< it precipitated 

 by saturation of the liquid with ammonium sulphate. The validity of this 

 reaction has been called in question. It has been pointed out that, although 

 the primary proteoses are readily precipitated by this salt, the deutero-pro- 

 teoses, under certain circumstances at least, are not precipitated, and cannot 

 therefore be distinguished or separated from the so-called " true peptone-." We 

 must await further investigations before attempting to come to any conclusion 

 upon this point. It is well to bear in mind that the change from ordinary 

 proteid to peptone evidently takes place through a number of intermediate steps, 

 and the word peptone is meant to designate the final product. Whether this 

 final product is a chemical individual with properties separating it from all the 

 intermediate stages is perhaps not yet fully known, but, provisionally at least, we 

 may adopt Kiihne's definition, outlined above, of what constitutes peptone, as it 

 seems to be generally accepted in current literature. Peptones are characterized 

 by their diffusibility, and this property is also possessed, although to a less 

 marked extent, by the proteoses. Recent work by Chittenden, 1 in which he 

 corroborates results published simultaneously by Kiihne, shows the following 

 relative diffusibility of peptones and proteoses. The solutions used were approx- 

 imately 1 per cent. ; they were dialyzed in parchment tubes against running 

 water for from six to eight hours, and the loss of substance was determined 

 and expressed in "percentages of the original amount. Proto-proteose gave a 

 loss of 5.09 per cent.; deutero-proteose, 2.21 per cent.; peptone, 11 per cent. 

 Rennin. — In addition to pepsin the gastric secretion contains an enzyme 

 that is characterized by its coagulating action upon milk. It has long been 

 known that milk is curdled by coming into contact with the mucous membrane 

 of the stomach. Dried mucous membrane of the calf's stomach, when stirred 

 in with fresh milk, will curdle the latter with astonishing rapidity, and this 

 property has been utilized in the manufacture of cheese. Hammarsten discovered 

 that this action is due to the presence of a specific enzyme that exists ready 

 formed in the membrane of the sucking-calf's stomach, and which is present 

 in a preparatory form (rennin-zymogen) in stomachs of all mammals. This 

 enzyme has been given several names; rennin seems preferable to any other, 

 and is the term most commonly employed. Rennin may be obtained from 

 the stomach by self-digestion of the mucous membrane or by extracting it 

 with glycerin. Such extracts usually contain both pepsin and rennin, but the 

 two have been separated successfully, most easily by the prolonged action of 

 a temperature of 40° C. in acid solutions, which destroys the rennin, but not 

 the pepsin. Good extracts of rennin cause clotting of milk with great rapidity 

 at a temperature of 40° C, the milk (cow's milk), if undisturbed, setting first 

 into a solid clot, which afterward shrinks and presses out a clear yellowish 

 liquid, the whey; with human milk, however, the curd is much less linn, 

 being deposited in the form of loose flocculi. The whole process resembles the 

 clotting of blood not only in the superficial phenomena, but also in the 

 character of the chemical changes. Briefly, what happens is that the rennin 

 1 Journal of Physiology, 1893, vol. xiv. |». 502. 



