DIGESTION 187 



After digestion has progressed for one to two days another 10 c.c. of liquor 

 pancreaticus may be added. 



The products of digestion in one case should be examined, say, after six 

 hours' digestion, and in another case after thirty-six hours' digestion or more. 

 The digestive products should then be searched for ; the early products of 

 digestion (alkali-albumin, deutero-proteose, &c.) will become less abundant 

 with the length of time that digestion has been allowed to progress, and the 

 later products (peptone, leucine, tyrosine, tryptophane, &c.) will become more 

 abundant. The methods for testing most of these substances have been 

 already given. The following are the tests for tryptophane, leucine, and 

 tyrosine : 



(a) Tryptophane. Add a few drops of bromine water ; a violet colour is 

 produced. 



(b) Leucine and Tyrosine. i. Examine microscopical specimens of these. 

 The deposit generally found in rather old specimens of Benger's liquor pan- 

 creaticus will be a convenient source of these crystals. 



ii. To some of the pancreatic digest add Millon's reagent and filter off 

 the precipitated protein. Boil the filtrate, and the presence of tyrosine is 

 indicated by a red colour. If tyrosine is abundant the red colour appears 

 without boiling. Leucine does not give this test. 



iii. Faintly acidify another portion of the filtered digest with acetic acid 

 and boil ; if any protein matter is still undigested it will be thus coagulated 

 and can be filtered off. Reduce the filtrate to a small bulk until it begins to 

 become syrupy. Leave overnight in a cool place, and crystals mainly of 

 tyrosine will separate out. Filter these off through fine muslin, and 

 evaporate down the filtrate to the consistency of a thick syrup ; leave this 

 overnight again, and a second crop of crystals, forming a skin on the surface 

 and consisting mainly of leucine, w r ill have separated out. 



7. Zymogen Granules. Examine microscopically, mounting in aqueous 

 humour or serum (or in glycerin after treatment with osmic acid vapour), 

 small pieces of the pancreas, parotid, and submaxillary glands in a normal 

 guinea-pig, 1 and also in one in which profuse secretion had been produced by 

 the administration of pilocarpine. 



Note that zymogen granules are abundant in the former, and scarce in 

 the latter, being situated chiefly at the free border of the cells. 



Extremely good, though not permanent, microscopic specimens may be 

 obtained by teasing in a 33-per-cent. solution of caustic potash. 



1 The guinea-pigs should be killed by bleeding, and the blood collected and 

 defibrinated, and utilised for the preparation of oxyhsemoglobin crystals. This 

 will give students an opportunity of seeing the exceptional form (tetrahedra) in 

 which the blood-pigment of this animal crystallises. 



The three methods of obtaining crystals described on p. 112 all give good results. 

 If amyl nitrite is used instead of ether in the third method, crystals of methasmo- 

 globin are obtained. 



