8 Neurological Technique 



fixing agent, it should either be of such a high per cent. (96 per 

 cent, to absolute), and with such small pieces of tissue, that 

 fixation of the cell structures may be accomplished before they 

 become distorted and the cell as much shrunken as results from 

 the longer period of diffusion currents involved in the action of 

 the intermediate grades ; or, it should be of such a low grade 

 (33^ per cent.) that the resulting diffusion currents are weak 

 enough to allow a more nearly balanced action between the 

 exosmotic and endosmotic currents. Weak alcohol thus causes 

 little or no shrinkage. However, because it has a dissociating 

 effect upon the tissue, and also because it denies the advantages 

 of rapid fixation, it is seldom used except where dissociation is 

 desired. 



The more important uses of alcohol in microscopic technique 

 are (i) washing the tissues after fixation by other fluids; (2) 

 hardening the tissues after fixing fluids which do not give the 

 required rigidity; (3) preserving the tissues after washing or 

 hardening; and (4) dehy.dration of tissues already fixed. 



Washing. — The surplus fixing fluid should always be removed 

 from the tissue before proceeding further. When fixing fluids 

 are used which at the same time harden the tissue, such as fluids 

 containing potassium bichromate or corrosive sublimate, much of 

 the surplus fluid may be washed out with water. The washing, 

 however, is always completed with alcohol. Alcohol, in almost 

 every case, is a better solvent than water, and also in it the 

 washing may be safely prolonged, since it preserves as well as 

 washes. In 70 per cent, or 80 per cent, alcohol, pieces of tissue 

 may remain indefinitely. The only exceptions to this rule are in 

 case of a few of the special methods where, in order to get the 

 best results, it has been found necessary that the preparation 

 be completed within a specified time. 



Hardening. — Material fixed in certain of the strongly acid 

 mixtures, for example those in which picric acid is the principal 

 ingredient, becomes softened or even macerated if washed in 

 water. Such material is transferred directly to alcohol, which, 

 while washing out the fluid, completes the process by gi/ing the 



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