32 METHODS OF CULTIVATION OF BACTERIA 



Preparation of Meat Extract. 



The flesh of the ox, calf, or horse is usually employed. 

 Horse-flesh has the advantage of being cheaper and containing 

 less fat than the others ; though generally quite suitable, it has 

 the disadvantage for certain purposes of containing a larger 

 proportion of fermentable sugar. The flesh must be freed from 

 fat, and finely minced. To a pound of mince add 1000 c.c. 

 distilled water, and mix thoroughly in a shallow dish. Set 

 aside in a cool place for twenty-four hours. Skim off any fat 

 present, removing the last traces by stroking the surface of the 

 fluid with pieces of filter paper. Place a clean linen cloth over 

 the mouth of a large filter funnel, and strain the fluid through 

 it into a flask. Pour the minced meat 

 into the cloth, and, gathering up the 

 edges of the latter in the left hand, 

 squeeze out the juice still held back in 

 the contained meat. Finish this expres- 

 sion by putting the cloth and its contents 

 into a meat press (Fig. 6), similar to that 

 used by pharmacists in preparing extracts ; 

 thus squeeze out the last drops. The 

 resulting sanguineous fluid contains the 

 soluble albumins of the meat, the soluble 

 salts, extractives, and colouring matter, 

 chiefly haemoglobin. It is now boiled 

 FIG. 6. Meat press. thoroughly for two hours, by which pro- 

 cess the albumins coagulable by heat are 



coagulated. Strain now through a clean cloth, boil for another 

 half-hour, and filter through white Swedish filter paper (best, 

 C. Schleicher u. Schull, No. 595). Make up to 1000 c.c. with 

 distilled water. The resulting fluid ought to be quite transparent, 

 of a yellowish colour without any red tint. If there is any red- 

 ness, the fluid must be reboiled and filtered till this colour dis- 

 appears, otherwise in the later stages it will become opalescent. ' 

 A large quantity of the extract may be made at a time, and 

 what is not immediately required is put into a large flask, the 

 neck plugged with cotton wool, and the whole sterilised by methods 

 B (2) or (3). This extract contains very little albuminous matter, 

 and consists chiefly of the soluble salts of the muscle, certain 

 extractives, and altered colouring matters, along with any slight 

 traces of soluble proteid not coagulated by heat. It is of acid re- 

 action. We have now to see how, by the addition of proteid and 

 other matter, it may be transformed into proper culture media. 



