212 HISTOLOGICAL PREPARATIONS 



the stone should not be oiled, but simply moistened with a little water or 



better still with the following solution : 



Distilled water, - - 50 c.c. 



Alcohol (95 per cent.), - - 50 



Glycerin, ..... - 50 



After use the razor should be dried on a piece of soft rag, lightly stropped, 

 and returned to its case. 



To cut sections embedded in paraffin the blade of the razor should be dry 

 and be placed obliquely to the tissue. The sections, as they are cut, should 

 be picked up from the razor with a pair of fine forceps or a piece of silk paper, 

 never with a needle or scalpel or other similar instrument which might 

 damage the cutting edge of the razor. 



2. Freezing methods. 



Though frozen tissues cannot be cut so thin as tissues embedded in paraffin, 

 the freezing method has the advantage that sections can be cut in a very short 

 time, and can be stained in a variety of ways ; and hence is of particular 

 value for purposes of rapid diagnosis. 



Only tissues which have been previously fixed should be cut by the freezing 

 method. Formalin (10 per cent.) is perhaps the best for the purpose (p. 189), 

 as tissues can be frozen without any further treatment. Tissues fixed by 

 other methods should be washed and then put in formalin for a few hours. 

 Tissues for frozen sections should not be more than 5-6 mm. thick. 



Microtomes. The simplest type for frozen sections is a rocking microtome 

 or a Minot. Place the tissue wet with formalin on the carrier of the micro- 

 tome and direct a jet of methyl chloride on to it until it is firmly frozen to 

 the carrier, then adjust the latter to the microtome and cut the sections. 



Of microtomes specially arranged for cutting frozen sections the best are 

 those of Becker and Miller, in which the tissue is frozen by the decompression 

 of liquid carbonic acid. The tissue is placed in an hollow carrier connected 

 by an iron tube to a cylinder of car"bonic acid, and when arranged in place 

 on the microtome is frozen by simply turning on the tap of the cylinder. 

 When the tissue is frozen the gas is turned off and the sections cut. If the 

 sections show a tendency to tear, it is because the tissue has been frozen 

 too hard, in which case it must be left for a few seconds. 



Transfer the sections to ordinary water in which they will uncurl ; when 

 uncurled they are ready for staining. 



3. Paraffin embedding methods. 



A. Xylol method. Method recommended. The pieces of tissue after being 

 fixed in the manner described in Chapter XL are treated as follows: 



1. Dehydrate carefully in absolute alcohol or acetone for 24 hours or 

 thereabouts. 



2. Transfer to xylol. 



Very small pieces (1-3 mm.) for - - 30-60 minutes. 



Small pieces (3-5 mm.), - 2 hours. 



Medium-sized pieces (5-10 mm.), - 3-4 



Thick pieces (10 mm. or more). 4-5 ,, 



In the case of the last it is as well to change the xylol once or twice to make quite 

 sure that all traces of alcohol will be removed. 



3. After dehydrating, transfer to a mixture of xylol and paraffin melting 

 at 35 C. Such a mixture can be made as follows : 



Paraffin 1 (melting point 50 C.), .... 10-15 grams. 



Xylol, - - 30 c.c. 



1 For embedding, the paraffin sold by Dumaige of Paris is recommended. 



