20 EXERCISES IN THE PREPARATION OF TISSUES. 



by tinging the last alcohol with the stain. Those treated with borax 

 carmine are placed in acid alcohol (17) for 24 hours, this makes the red 

 brighter, and thence are passed through increasing strengths of alcohol 

 as in the former case. 



Section Cutting 1 . 



The razor must have a keen straight edge and a stiff back, 

 and must not be hollow ground. 



Hand cutting-. Holding the piece of tissue in the left hand, 

 between the thumb and forefinger, and the razor in the right, guide 

 the razor upon the forefinger of the left and cut thin slices by 

 steady heel to point strokes of the razor. Keep the blade of the 

 razor and the surface of the tissue moistened with spirit or other 

 fluid in which the tissue happens to be preserved. The piece of 

 tissue may be held between pieces of hardened liver or elder pith. 



As tissues generally require support, owing to the delicateness of 

 their structure, it is usual to embed them in a material which will 

 permeate them. In order to make even and thin sections a microtome 

 is employed. Three principal methods are in use as follows : 



Employ the prepared tissues of the frog in illustration of the 

 following methods, thus : 



Tissues (a) and (b) are to be cut in paraffin. 

 (a) gum. 



(c) celloidin. 



Embedding- and cutting in paraffin.' 



Transfer the pieces of tissue into fifteen times their volume of absolute 

 alcohol for 24 hours in order to complete their dehydration. Then 

 "clear" them by a corresponding immersion in toluene. This is for 

 general purposes the best intermediate solvent. This step is required 

 because alcohol will not mix with paraffin. 



When cleared (24 hours) they are ready for embedding. Wipe off the 

 surface toluene and immerse the pieces in melted paraffin (melting point 

 55 C.) for one hour to an hour and a half in the embedding bath. 



1 Very delicate tissues require to be guarded from the collapse of cavities which is 

 apt to occur with this process by a more gradual transference to paraffin, this is accom- 

 plished by transferring from alcohol to chloroform, and when the latter has quite 

 replaced the alcohol, the tissue sinks in it. Then place in a fresh quantity of 

 the fluid and add shavings of solid paraffin until no more dissolves. Allow the 

 fluid to thicken by evaporation, warm to the temperature of the embedding bath, 

 and transfer to pure melted paraffin in the latter. 



