150 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



solution of sodium carbonate to which the white of an egg has been added 

 every ten hours. Note the odour due to putrefaction. Another preparation 

 has been similarly made, except that thymol has been added to prevent de- 

 composition. These should be got ready by the demonstrator. 



Filter some of the extract and examine for leucine and tyrosine aa- 

 foUows : 



(a) To some of the liquid add Millon's reagent and filter off the pre- 

 cipitated proteid. Boil the filtrate. The presence of tyrosine is indicated 

 by a red colour. If tyrosine is abundant the red colour appears without 

 boiling, 



(6) Faintly acidify another portion of the filtered extract with 2-per-cent. 

 acetic acid and boil ; filter off the proteid thus coagulated ; reduce the filtrate 

 to a smaU bulk by evaporation on the water-bath at the boUing temperature. 

 Examine a drop microscopically for crystals of leucine and tyrosine. Treat 

 the remainder with excess of alcohol, to precipitate the albumoses and 

 peptones, and again filter. Concentrate the filtrate on the water-bath till it 

 becomes sticky from the presence of leucine. Examine some of the concen- 

 trated fluid with the microscope ; leucine will be found in crystalline 

 spheroidal cliunps. 



Examine microscopic specimens of leucine and tyrosine which have been 

 prepared by the demonstrator.^ 



4. 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 "^ and also in one in which profuse secretion had been produced by 

 the administration of pilocarpine. 



Note that zjTHOgen 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. 



' The deposit often found in rather old specimens of Benger's liquor pancrea^ 

 ticus will be found to consist of leucine and tyrosine crystals. 



^ 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. 89 all give good results.. 

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

 globin are obtained. 



