640 TECHNIQUE 



useful, for in it tissues may be kept for a long period without deteri- 

 oration 



THE DISSOCIATION OF TISSUES 



It is frequently desirable to dissociate tissues, to a certain extent, 

 into their component elements, prior to microscopical examination. 

 This is accomplished by teasing, or by the solvent action of rela- 

 tively strong acids or alkalies. 



For teasing, minute bits of tissue are torn off by the aid of 

 needles or scissors and carried to a clean slide, where they are to be 

 kept always moistened with normal saline solution or other isotonic 

 fluid. For their further manipulation a dissecting microscope is 

 useful, though not essential. Two sharply pointed needles, mounted 

 in wooden handles, are to be used. 



The bit of tissue is pinioned with the needle held in the left 

 hand, and with that in the right the tissue is gently torn by a 

 rhythmic combing motion, being very careful to avoid squeezing the 

 tissue between the needle and the slide. With a little practice 

 bundles of fibres, groups of cells, etc., are readily separated suffi- 

 ciently to be studied under moderate magnification. During the 

 teasing, the bits of tissue should be kept well moistened, and are 

 to be frequently inspected under low magnification to determine 

 the progress of the operation. When satisfactorily prepared, a 

 cover glass may be applied, and the preparation examined under 

 higher magnification. 



In applying a cover glass care should be taken to permit one 

 edge of the cover to first touch the slide while being held at an 

 angle of 30 to 40. If the cover is then gently lowered into place, 

 the air is forced out before the advance of the fluid, and the many 

 air bubbles which would otherwise be included are not found in 

 the preparation. 



The method of teasing is particularly applicable to the study 

 of the connective, and peripheral nervous tissues. White fibres, 

 elastic fibres, fat cells, and nerve fibres are readily isolated in this 

 way. If desired, they may be stained by the addition of a drop of 

 a solution of methyl green, picro-carmin, etc. 



Chemical Dissociation. It is necessary to dissociate many tissues 

 by chemical means, either because of the firm union of the elements 

 composing the tissue or because they may be too delicate and 

 fragile to withstand the mechanical teasing. Epithelial cells, nerve 

 cells, and muscle fibres are readily prepared in this way. 



