XXXV 111 



INTRODUCTION. 



Sections to be cut in Ranvier's microtome are imbedded just as in Mr. Stirling's form, 

 with a paraffine and lard or other mixture. Elder-pith, however, may be used. The tissue is 

 packed in with dry elder-pith, and then the whole apparatus is placed in spirit, which causes 

 the pith to swell, and so to fix the tissue sufficiently firmly for a section to be made of it. 



FREEZING MICROTOMES. 



Dr. Rutherford's Freezing Microtome. This microtome (fig. 1 9) is essentially a modification 

 of Mr. Stirling's microtome. The hollow brass cylinder is surrounded with a large trough (G), 

 which is filled with a freezing mixture composed of equal parts of salt and ice. The trough is 

 provided with a tube (H) to permit of the outflow of the water resulting from the melting of 

 the ice. It is a great improvement to have the cutting- 

 table covered with a thick plate of glass, as this is not worn 

 away so readily as the brass. This microtome can be used 

 as an ordinary microtome, as well as for freezing. 



METHOD OF USING A FREEZING MICROTOME. 



Preparation of the Microtome. Clamp the instrument 

 to a table. Turn the screw (D) until the plug has descended 

 a good distance into the well, and to prevent the screw 

 freezing pour a little alcohol into the well and let it run 

 through the tube ; dry the well thoroughly, and smear a 

 little lard round the top of the plug, to prevent the escape 

 of any of the imbedding mixture between the plug and 

 the wall of the cylinder, which would interfere with the 

 action of the screw. 



Preparation of the Tissue to be Frozen.- The tissue has 

 first been properly hardened by one of the foregoing 

 methods. If it has been kept in alcohol, it must be 



placed in a large quantity of water to be changed several times during twenty-four hours, 

 to remove all the alcohol. It is then placed for twenty-four hours in gum-mucilage (B. P.). The 

 well is then filled with gum (B. P.), and equal quantities of powdered ice and salt are packed 

 into the trough with a piece of stick until the edge of the gum is observed to freeze, when the 

 piece of tissue is placed in the gum in the side of the well nearest the side from which the 

 person makes the sections, by means of a needle. The freezing process is continued until the 

 whole mass of gum becomes solid, the gum becoming of such a consistence that it cuts like a 

 piece of cheese (U. Pritchard). 



The Section Knife. The ordinary razor does not answer well for cutting sections with the 

 microtome. Its blade is too thin and too short. It is necessary to have a stout blade, seven 

 or eight inches long, with a thick straight back at least a quarter of an inch thick fixed in 

 a straight handle. The blade must be so strong that it cannot be bent whilst pushing it 

 along the glass plate. The surface next the glass plate ought to be slightly concave. The 

 surface of the knife ought to be moistened with gum solution, which prevents the sections 

 from curling up. In cutting, keep the back of the knife directed towards the operator, and 

 push the blade obliquely from tip to heel, guiding it with the thumb and forefinger of the left 

 hand through the tissue. Each section as it is made must be floated off into a bowl of water 

 with the aid of a large camel-hair brush. (See p. xl for how to preserve them.) 



Fig. 19. DR. RUTHERFORD'S FREEZING 

 MICROTOME. 



B, Plate of brass with aperture A leading 

 into the well of the Microtome ; c, Lower 

 end of cylinder in which the screw I) 

 works, moving a brass plug up or down ; 

 G, Trough to hold freezing mixture ; n, 

 Exit tube for water ; F, Clamp to fix the 

 instrument to a table. 



