THE MICROTOME AND SECTIONING. 



37 



A drop of fluid (physiologic saline solution, water, etc.) is placed upon 

 the knife, in which the section thaws out and spreads. A better and 

 more rapid method of freezing tissues consists in the use of compressed 

 carbon dioxid, as recommended by Mixter. Cylinders containing about 

 twenty pounds of the liquid gas may be obtained from Bausch & Lomb, 

 who also make a small microtome designed for this purpose. In figure 7 

 is shown the lower third of a cylinder for compressed carbon dioxid 

 firmly fastened to a thick board, and connected by means of a short piece 

 of strong rubber tubing with the freezing box of the microtome. The 

 handle of the escape valve is from 8 to 10 inches long, so that the 

 quantity of escaping gas may be readily controlled. The pieces of tis- 

 sue are placed on the freezing box of the microtome and the escape valve 

 slowly opened until a small quantity of the gas escapes. Small pieces of 

 tissue are frozen in about thirty seconds to a minute ; tissues taken from 

 alcohol should be washed for a short time in running water before freez- 

 ing. A strong razor may be used for cutting sections ; or better, a well- 

 sharpened blade of a carpenter's plane, as suggested by Mallory and 

 Wright. Sections are transferred to distilled water or normal salt solu- 

 tion, and if fixed may be stained at once. Sections of fresh tissue 

 should be taken from the normal salt solution and transferred to a fixing 

 fluid. 



Bardeen has devised a microtome to be used with compressed carbon 

 dioxid, which presents many advantages. It admits of better control of 

 the temperature of the freezing stage and there is less carbon dioxid wasted 

 than with other instruments of this type. It freezes almost instantane- 

 ously, since the expanding carbon dioxid is caused to pass through a 

 spiral passage contained in the freezing chamber. In this apparatus the 

 microtome is attached to the steel cylinder containing the carbon dioxid. 

 It is impossible to cut thin sections with a knife that is not sharp, 

 or with one that is nicked. A few directions as to sharpening a micro- 

 tome knife may therefore not be out of place. For this purpose a good 



Fig. 7. Apparatus for cutting tissues frozen by carbon dioxid. 



Belgian hone is used, which should be moistened or lubricated with filtered 

 kerosene oil or with soap as necessity demands. While sharpening the 

 knife it is grasped with both hands with one by the handle, with the 



