SECTION-CUTTING BY THE FREEZING METHOD 21? 



thick and syrupy solution of gum arabic; if time is no object 

 it is a great advantage to soak them in this for several hours. 

 A block is then placed on the corrugated plate of the micro- 

 tome and frozen by means of the ether spray which impinges 

 upon it. When the mass is nearly frozen a section is taken 

 off by means of a razor which is ground flat on one side, or 

 the special knife which may be obtained with the apparatus; 

 it is better to moisten the upper surface of the knife with a 

 little of the gum. The section is carefully removed with a 

 camers-hair brush and placed in a large vessel of clean water, 

 so that the gum may be dissolved out of it, and is then ready 

 for staining-. The block is then raised by means of a very 



FIG. 41.' CATHCART'S MICROTOME ARRANGED FOR CUTTING FROZEN SECTIONS. 



slight turn of the large milled head under the apparatus, 

 and another section cut. 



The mass must not be frozen too hard; if this has been 

 the case, the necessary thawing will be hastened by gently 

 breathing on the block. If it thaws too much, a few squeezes 

 of the bellows will bring it to the proper consistency. 



For extremely rapid work, the best method of freezing is 

 by the use of liquid carbon dioxide : this, however, is diffi- 

 cult in private practice, owing- to the unportability of the 

 cylinders. The best method is to use ethyl chloride or 

 anaestile in metal cylinders, such as are used for local 

 anaesthesia. The best method is as follows : Place a few 

 drops of water on the plate of the microtome, and freeze it 



