232 PRACTICAL PHYSIOLOGY 



be prepared by saturating gastric juice with ammonium sulphate, which 

 precipitates it. The methods employed for obtaining ferment from the 

 gastric mucosa after death yield a still more impure product, on account 

 of the ferment adhering to the proteoses, etc., which are always present in 

 the final precipitate. 



Prior to its secretion, pepsin exists in an inactive form as granules in the 

 gland cells of the stomach mucosa. This precursor or zymogen is called 

 pepsinogen. It differs from pepsin in that alkali does not destroy it, whereas 

 alkali destroys pepsin. 



The most favourable conditions for the action of pepsin may be 

 studied in the test tube as described in the following experiments : 



The Action of the Gastric Juice. The most convenient protein for 

 studying the action of pepsin is blood fibrin which has been very 

 thoroughly washed with boiling acidulated water so as to remove 

 all impurities. Cubes of coagulated egg white may also be employed, 

 but they digest more slowly than fibrin. 



EXPERIMENT V. Label six test tubes A, B, C, D, E, F, and 

 place a small piece of fibrin in each. Half fill A with water, B 

 with 0-2 per cent. HC1, C with water and a few drops of peptic 

 extract, D with 0-2 per cent. HC1 and a few drops of peptic extract, 1 

 E same as D, but boil the mixture, and F with 1 per cent, sodium 

 carbonate solution and a few drops of the peptic extract. 



Place all these in a water bath kept at body temperature 

 (37-38). Observe that in A and C the piece of fibrin remains 

 unchanged, whereas in B, D, and E, which all contain 0-2 per cent. 

 HC1, it becomes swollen and transparent. In F, which contains 

 alkali, it does not swell. 



EXPERIMENT VI. After about half an hour, remove a sample of 

 the contents of any of the tubes containing acid, colour it faintly 

 with a drop or two of litmus solution, and then carefully neutralise 

 with weak sodium carbonate solution (1 part 1 per cent, sodium 

 carbonate -f- 2 parts of water). A precipitate of acid meta-protein 

 is usually produced (for reactions, see proteins, p. 202). 



The first stage in gastric digestion of proteins consists, therefore, 

 in the production of acid meta-protein by the weak HC1. 



EXPERIMENT VII. Remove a sample of the contents of D and 

 apply the following tests : (a) The Biuret reaction rose-pink 

 colour ; (6) Add nitric acid (cone.) white precipitate, which clears 

 up on heating and returns on cooling ; (c) Add a few drops of a 

 saturated solution of salicyl-sulphonic acid. A white precipitate 

 results which disappears on heating and returns on cooling. These 

 results show us that proteoses have been produced (see p. 203). 



Gastric juice can also curdle milk. This action is usually attri- 

 buted to the ferment rennin, but it is probable that rennin and 

 pepsin are identical, as proteolytic ferments always have a rennin 

 action, and the proteolytic activity of a given ferment is propor- 

 tional to its rennin activity. 



EXPERIMENT VIII. Prepare five test tubes, a, b, c, d, e. Into 



1 Use larger quantities of fibrin and fluid in this test tube, because the 

 products of digestion will be required for succeeding experiments. 



