GENERAL INSTRUCTIONS. 



The following methods are given in such detail in order that 

 they may be followed by the student who has not had much 

 experience in histological technique. In the directions the vari- 

 ous steps and manipulations are such as have been found to 

 work best in the actual application of the methods in the 

 laboratory. 



The methods given have been devised for the preparation and 

 study of vertebrate nervous tissue. In most cases they are 

 especially applicable to human material and that of the higher 

 mammals generally. 



1. In removing the nervous system care should be taken that 

 the portions exposed do not become dry, and that the parts 

 removed are not crushed or stretched. The first object may be 

 attained by exposing only a small portion of the nervous system 

 at a time, or by covering the parts exposed with skin or paraffin 

 paper. When this is not convenient, the exposed parts may be 

 kept moist with normal salt solutions (0.75 per cent. NaCl) 

 used very sparingly. The second object may be accomplished 

 by using only sharp instruments, and employing scissors when- 

 ever practicable. 



m case of very young animals, or foetuses, it is best to fix 

 the pieces of central nervous tissue before removing them com- 

 pletely from the surrounding tissues. 



2. Always use an abundance of any fluid. It is safer to use 

 too much than not enough. 



3. Always use as small pieces of tissue as the purpose for 

 which they are taken will allow. 



4. The time during which the tissue should be subjected to 

 the action of a reagent depends, of course, upon the size of the 

 piece, the nature of the material, and the character of the 

 reagent. When a choice of times is offered in the directions, 



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