BY DR. LAUDER BRUNTON. 489 



of the hits of fibrin become translucent. If the filtrate is acid, 

 a bit of fibrin is thrown into it; if it swells up, it is allowed to 

 stand, if not, acid is added as before directed till it does swell ; 

 the digestion is then allowed to go on at the temperature of 

 the room, and the result observed. 



The residue which remains on the filter is introduced into a 

 beaker covered with 0.1 per cent. Ii3*drochloric acid, and placed 

 in the water-bath at 40 C. for an hour and a half or two hours, 

 or allowed to stand 24 hours at the temperature of the room, 

 with frequent stirring. It is then filtered, and the filtrate used 

 in the same manner as before. The reason wh3 T Briicke recom- 

 mends that the wateiy extract should be tested separately from 

 the acid extract, is that by this means pepsin already excreted 

 from the peptic cells can be distinguished from pepsin still con- 

 tained in them, inasmuch as the former is easily taken up by 

 water alone, while the latter is taken up with difficulty by 

 water, but easily by dilute acid. This process has also the ad- 

 vantage that when soluble albuminous bodies are present in 

 au3 r quantity, they are, for the most part, removed by the 

 watery extract. If neither of these objects is of importance, 

 the substance may be at once treated with dilute hydrochloric 

 acid, and when it is small, as, for example, the salivary glands 

 of insects, it ma}^ be at once thrown with a bit of fibrin into 

 dilute hydrochloric acid, and digestion looked for. If a fluid 

 is to be examined it must be filtered, and the filtrate and residue 

 treated as above directed for solids. 



Pepsin Test with WJiite of Egg. White of egg is more readily 

 got than fibrin, but it dissolves mere slowly, so that the test 

 takes a longer time. Hard boiled white of egg, cut into dice, 

 may be left fora long time in dilute hydrochloric acid without 

 undergoing an}' changes, but the coagulura which is produced 

 by boiling white of egg diluted with water undergoes partial 

 solution pretty rapidly. The free alkali contained in white of 

 egg is the cause of this difference in its behavior when prepared 

 in these different ways, and the inconstancy of its amount ren- 

 ders it difficult to determine what degree of acidity must be 

 given to the liquid. To obviate this, add acetic acid to white 

 of egg diluted with water until it turns blue litmus paper violet, 

 but not red. Filter from the precipitate; test the reaction of 

 the filtrate again, and correct it if necessary. Then coagulate 

 it in the water-bath, wash it with water, and use it like fibrin, 

 but use an acid of 0.15 per cent. If pepsin is present, diges- 

 tion will go on just as with fibrin. The acid alone will not dis- 

 solve the albumin for many days. 



126. Theory of Pepsin Digestion. It has already been 

 seen that neither pepsin alone, nor hydrochloric acid alone, 

 will digest. C. Schmidt supposes they do so when mixed, by 

 forming a compound acid pepto-hydrocliloric acid. He thinks 



