FREEZING METHOD 95 



possible sections of the organ or tissue in question. To 

 prepare these, the tissues are first ' hardened ' by the appli- 

 cation of various reagents, and afterwards cut into sections 

 with some form of microtome. 



Freezing Method. — This is a very rapid method of preparing 

 sections. This is done by means of an instrument known 

 as a freezing-microtome. 



A small piece of the fresh tissue is laid on the roughened 

 plate, and then frozen hard by means of the ether-spray 

 apparatus. Sections are now cut of the required thickness 

 by means of the razor-blade attached to the apparatus. 

 This freezing process frequently destroys delicate tissues, 

 owing to the ice which forms bursting the cells of the 

 tissue, so it is usual to harden the tissues before cutting 

 the sections. 



Hardening of Tissues. — The most satisfactory hardening 

 reagent for bacteriological purposes is absolute alcohol. 

 Small pieces of the tissue are cut about a cubic centimetre 

 square ; these are immersed for about forty-eight hours in 

 absolute alcohol, which is changed frequently. A good 

 plan is to place the alcohol in a wide-mouthed bottle, in 

 the cork of which are fixed several needles. The pieces of 

 tissue are placed on the needles in such a manner that, 

 when the cork is fixed in the mouth of the bottle, the 

 pieces of tissue are just beneath the surface of the alcohol. 

 The alcohol gradually abstracts the water from the tissue, 

 and as that containing the water sinks to the bottom, fresh 

 alcohol constantly comes in contact with the material. 

 Tissues containing much water are, of course, more difficult 

 to harden than those containing little. 



Another method of hardening tissues is to soak them for 

 thirty minutes in 5 per cent, solution of mercury bichloride 

 (corrosive sublimate), kept at about 70° C. ; after which 

 treatment they are transferred to alcohol, when, after re- 



