256 THE TISSUES AND MECHANISMS OF DIGESTION. 



readily coagulated at a temperature of 75 C. This new proteid matter of 

 which we are speaking is soluble in neutral solutions, indeed in distilled 

 water, and can under no circumstances be coagulated by heat. 



Upon examination we find that the new proteid matter thus left in solu- 

 tion consists of at least two distinct proteid bodies. If to the solution ammo- 

 nium sulphate be added, part of the proteid matter is precipitated while part 

 is still left in solution. The proteid body thus thrown down is called albu- 

 mose (there are several varieties of albumose, but these need not now detain 

 us). It approaches albumin in nature by reason of the fact that it will not 

 diffuse through membranes ; that it differs, however, widely from that proteid 

 is shown by its solutions not coagulating on boiling. The body which is not 

 thrown down by ammonium sulphate is called peptone; it differs from albu- 

 mose in being diffusible, for it will pass through membranes. The diffusion 

 is not nearly so rapid as that of salts, sugar, and other similar substances ; 

 indeed solutions of peptones may be freed from salts by dialysis. But it is 

 very marked as compared with that of other proteids ; these pass through 

 membranes with the greatest difficulty, if at all. Peptone is insoluble in 

 alcohol, and may be precipitated from its solutions by the addition of an 

 adequate quantity of this reagent ; but for this purpose a very large excess 

 of alcohol is needed, otherwise much of the peptone remains in solution. It 

 may be kept under alcohol for a long time without undergoing change, 

 whereas other proteids are more or less slowly coagulated by alcohol. A 

 useful test for peptone is furnished by the fact that a solution of peptone, 

 mixed with a strong solution of caustic potash, gives on addition of a mere 

 trace of cupric sulphate in the cold a, pink color, whereas other proteids give 

 a violet color. In applying this test, however, care must be taken not to add 

 too much cupric sulphate, since in that case a violet color, deepening on 

 boiling, that is, the ordinary proteid reaction (see 15), is obtained. 



There are reasons for thinking that there are several kinds, or at least 

 more than one kind, of peptone ; but we may for the present regard the sub- 

 stance as one. For a long time albumose was confounded with peptone, and 

 many of the commercial forms of "peptone" consist largely of albumose; 

 indeed, the two are closely allied and have many reactions in common, the 

 most striking differences being that peptone is diffusible, while albumose is 

 not, or hardly at all, and that peptone is not, like albumose, precipitated by 

 ammonium sulphate. The amount of albumose appearing in a digestion 

 experiment, relative to the amount of true peptone, depends on the activity 

 of the juice, and other circumstances. We may regard albumose as a less 

 complete product of digestion than peptone. 



The precipitate thrown down by neutralization after the action of gastric 

 juice on egg- or serum-albumin resembles, in its general characters, acid- 

 albumin. Since, however, it probably is distinguishable from the body or 

 bodies produced by the action of simple acid on muscle or white of egg, it is 

 best to reserve for it the name of parapeptone, which was originally applied to it. 



Thus the digestion by gastric juice of solutions of egg-albumin or serum- 

 albumin results in the conversion of all the proteids present into peptone, 

 albumose, and parapeptone, of which the first may be considered as the final 

 and chief product, and the other two as intermediate products, occurring in 

 varying quantity, possibly not always formed, and probably of secondary 

 importance. When fibrin, either raw or boiled, or any form of coagulated 

 proteid is dissolved and seems to disappear under the influence of gastric 

 juice, the same products, peptone, albumose, and parapeptone make their 

 appearance. The same bodies result when myosin or any of the globulins 

 are subjected to the action of the juice ; and acid-albumin or alkali-albumin 

 is similarly converted into albumose and peptone. 



