36 THE MICROSCOPIC PREPARATION. 



If the tissues are well imbedded and not too hard, and if the knife 

 is sharp and properly adjusted, paraffin sections may be cut in such a way 

 that each succeeding section adheres to the preceding one, so that actual 

 ribbons of paraffin sections may be made. In order to do this, the knife 

 should be at right angles to the microtome. The paraffin block should be 

 trimmed in such a way that when clamped in the microtome ready for 

 cutting sections, the surface of the paraffin block facing the knife should 

 be exactly parallel to its edge, also to the opposite side of the block. In 

 other words, 2 sides of the paraffin block should be parallel to each 

 other and to the knife ; then if the paraffin is of the right consistency, 

 which must be ascertained by trying, the sections as they are cut will ad- 

 here to each other and form a ribbon. If the sections do not adhere to 

 each other it is quite probable that the paraffin is a little too hard. This 

 may often be remedied by holding an old knife or other metallic instru- 

 ment which has been heated in a flame near the two parallel surfaces for a 

 few moments. Care should be taken not to allow this instrument to touch 

 the paraffin. This is a very convenient and rapid way of cutting par- 

 affin sections. To facilitate the cutting of a paraffin possessing a rela- 

 tively low melting point in a room with a high temperature, the cooled 

 knife of Stoss may be used. This is so made that a stream of ice-water 

 may be passed through a tube running through the entire length of the 

 back of the blade. Paraffin sections may be cut in ribbons serial sec- 

 tions on an ordinary sliding microtome; for this purpose, however, the 

 "automatic rotary microtome" of Minot is especially recommended. 



Celloidin Sections. Before fastening the block of wood or vul- 

 canized fiber to which the celloidin blocks have been fixed in the clamp 

 on the microtome, the celloidin should be trimmed with a sharp knife 

 from the top of the block until the tissue is nearly exposed, care being 

 taken to leave a flat surface. The sides of the celloidin block are then 

 trimmed down, if necessary, to within about y 1 ^ of an inch of the tissue. 

 The block is now clamped in the microtome at such a level that it just 

 escapes the knife when drawn over it. The knife is placed at an angle of 

 about 45, or at even a greater angle. During the process of cutting, the 

 knife, as also the tissue, must be kept constantly moistened with So c / G 

 alcohol. This is perhaps most easily accomplished by taking up the 80% 

 alcohol with a rather large camel' s-hair brush and dipping this on the 

 celloidin block and on the knife. A number of rather thick sections are 

 cut until the knife touches the entire surface of the block or until the tis- 

 sue is well exposed. The sections may now be kept. The block is raised 

 20 to 15 microns, and the knife, which should be well moistened with 

 80% alcohol, is drawn over the block with a steady pull, not with a 

 jerk. The sections are transferred from the knife to distilled water. 

 This is perhaps most conveniently done by placing the ball of one of the 

 fingers of the left hand under the edge of the knife, in front of the sec- 

 tion, and drawing the section down onto the finger with the camel' s-hair 

 brush. The finger is then dipped into the distilled water when the sec- 

 tion floats off. If the sections can not be stained within a few hours after 

 they are cut, they are best transferred to a dish containing 80 y c alcohol, 

 in which they may be left until it is desired to stain them. 



The sliding microtomes may be provided with an arrangement for freez- 

 ing tissues a so-called freezing apparatus. This consists of a metal 

 plate on which the tissue is laid; an ether or rigolene atomizer plays upon 

 its lower surface, cooling and finally freezing the object, which is then cut. 



