BY DR. KLEIN. 101 



cork, by mechanical means. This is not advantageous on the 

 following grounds : Those tissues which are soft are so injured 

 by the pressure that their elements are in a completely un- 

 natural condition ; whereas, in the case of firm tissues, it is 

 quite easy to do without such assistance. 



Preparation of Sections by Freezing. For the pur- 

 pose of obtaining sections of tissue without any dislocation or 

 alteration of structure, the method of freezing is well adapted. 

 A freezing mixture is prepared by introducing alternately 

 small quantities of broken ice, or snow (not so advantageous), 

 and of finely powdered salt, into a large vessel, mixing the 

 two ingredients thoroughly after each addition. The tem- 

 perature should be determined by the introduction of a ther- 

 mometer. The object, which must be small, should be cut to 

 an oblong form, and placed on a flat cork, much wider than 

 itself. It must be pinned to this cork at the end opposite that 

 from which the sections are to be cut. In the case of a mem- 

 brane, the object must be folded, and fixed in the same way. 

 The whole is then placed in a platinum crucible, which has 

 been previously plunged into the freezing mixture. The cru- 

 cible must be at once covered, and a little of the freezing 

 mixture placed on the top of it. Tke section knife, which 

 must be sharp, is cooled by laying it on ice. As soon as it is 

 ascertained, by exploration with a needle, that the preparation 

 is firm enough, the knife is handed to an assistant, who wipes 

 it, and holds it in readiness. The cork is then taken out with 

 the forceps, and seized by the fingers of the left hand in such 

 a way that they do not come into contact with the preparation. 

 A succession of sections having been rapidly made, the num- 

 ber varying with the skill of the operator, the cork is replaced 

 in the crucible. The sections may be employed either for 

 immediate examination, or for teaming, or subjected to further 

 processes of preparation. As soon as the portion of tissue in 

 the crucible is again of the proper consistence, more sections 

 can be made. As regards the temperature which should be 

 employed, and the time during which the object should be 

 frozen, no definite rule can be given. It may be stated, in 

 general, that temperatures varying from 6 to 20 C. are 

 sufficient for all purposes. The time necessary for the attain- 

 ment of the proper degree of firmness is obviously dependent 

 on the temperature of the freezing mixture, on the thickness 

 of the object, and on the relative quantity of water it contains. 

 Accordingly the time is very variable, so that the proper 

 moment for removing the preparation can only be determined 

 by frequently repeated exploration ; by which means alone it 

 is" possible to avoid the risk of carrying the hardening too 



far a result which is alike prejudicial to the structure of the 



organ, and to the success of the section. 



