HISTOLOGICAL METHODS. 167 



unless the sections be very large, when it should be flat or 

 slightly convex. The concavity is convenient for carrying 

 a pool of spirit, over which the section floats ; fat floating is 

 essential. It is convenient to keep the back of the knife 

 towards the operator, and either to draw it steadily through 

 the tissue from heel to point, or to push it in the opposite 

 direction. The blade of the knife should not be too thin, 

 otherwise it tends to bend into the well. A considerable 

 amount of dexterity is required, but this is soon gained by 

 practice. 



305. When the microtome is used for freezing, the freez- 

 ing box requires to be covered with a non-conductor, such 

 as a thick jacket of felt, flannel, or gutta-percha. The 

 latter is much to be preferred, and an instrument having 

 this covering should be obtained. The plug (K, Fig. 59) 

 is unscrewed and oiled to prevent its becoming fixed during 

 the freezing. The tissue and an imbedding fluid are placed 

 in the well. If a watery fluid be employed, it becomes 

 crystalline when frozen, and is therefore badly adapted to 

 serve as the imbedding agent. 



306. The difficulty is, however, entirely overcome by 

 the method of imbedding in gum ( 301), as suggested to 

 me by my late assistant, Dr. Pritchard. A thick solution of 

 gum when frozen does not become crystalline, and can be 

 cut like cheese. If there be any alcohol in the tissue it 

 must be thoroughly removed by immersion in water for six 

 to twenty-four hours, according to the size of the tissue, 

 before it is placed in the gum. // is always advantageous 

 to allow the tissue to soak in the gum for twelve to twenty- 

 four hours before freezing, in order that the gum may permeate 

 every part of the tissue and prevent the formation of a crystal- 

 line condition within the frozen tissue. Thick gum solution 

 is poured into the well, and, when a film of ice has formed 

 at the periphery, the tissue should be introduced and held 

 against the advancing ice until it is fixed. In this way the 

 tissue may be secured in any position for the process of 

 section. A plate of cork with a weight on it is placed on 

 the microtome plate to prevent the entrance of heat, and 

 also of salt from the freezing mixture. 



