52 PRACTICAL HISTOLOGY [VII 



care should be taken to remove as much as possible of the clearing 

 agent, otherwise the Canada balsam in which the section is mounted 

 may remain a long time fluid. 



3. Ether Freezing Microtome. The fine nozzle of the ether 

 spray apparatus is easily stopped up by fine particles of dust. 

 To avoid this, see that the bottle to contain the ether is clean, 

 and filter into it the ether to be used. 



The piece of tissue should not be more than 3 to 4 mm. thick ; 

 for the upper surface of pieces thicker than this freezes slowly. 



Before beginning the ether spray, adjust the razor (see 2) so 

 that the edge just touches the top of the tissue as it lies on the 

 plate ; place a brush and dish of water in readiness. Then set 

 the ether spray going by means of the hand bellows, keeping the 

 second bag of the bellows just pressing against the net sur- 

 rounding it. The tissue should be frozen in about a minute, the 

 time varying with the temperature of the room. If the freezing 

 does not begin rapidly, look at the spray ; if there is not a good 

 jet, the nozzle of the apparatus probably requires cleaning out. 



As soon as the tissue is frozen, begin cutting sections as with 

 the ice and salt microtome. Immediately the gum on the plate 

 at the edge of the tissue begins to lose its dense white colour, 

 cease cutting sections, and ply the ether spray to re-freeze. 

 Then cut more sections. 



When the sections are cut, clean the razor and the microtome 

 plate. 



4. Cutting Fresh Tissues. A piece of tissue fresh from the 

 body may be placed direct on the microtome plate and frozen. 

 It is best to cut it a few degrees only below freezing-point. The 

 ice rapidly blunts the razor, and ice crystals are apt to distort 

 and tear delicate structures. This can be more or less avoided 

 by placing the piece of tissue for 5 to 15 minutes in dilute gum, 

 this however causes some changes in the tissue cells. 



a. Cut sections of e.g. a piece of rat's kidney. Transfer the 

 sections to salt solution 0'9 p.c. and with a brush gently unfold 

 them. Dip a slide obliquely under a section, with a needle hold 

 one corner of the section on the slide, lift up the slide, if necessary 

 pull out the edges of the section so that it lies flat and unfolded, 



