290 NORMAL HISTOLOGY. 



sinks to the bottom of the jar owing to its greater specific gravity, 

 and is at once taken up by the copper sulphate. 1 



It will be seen that this method not only fixes the tissues, but also 

 quickly dehydrates them. The real dehydrating-agent is, however, 

 the cupric sulphate, the alcohol serving merely as a vehicle for 

 conveying the water from the specimen to that salt. If the pieces 

 of tissue are small, not over 5 mm. thick, they will be hardened by 

 remaining in the absolute alcohol over night, and mounted sections 

 may be ready for examination by the next afternoon. 



8. Fixation by Boiling. — Throw small pieces of the tissue, not 

 larger than 1 cm., into boiling 0.75 per cent, salt solution. Keep 

 them at the temperature of boiling for two minutes. Then throw 

 them into cold water. They may then be cut with the freezing- 

 microtome, or may be placed in 70 per cent, alcohol for hardening. 

 This method is excellent for the detection of albuminous exudates 

 within the tissues, but it causes so much shrinkage that it is not 

 useful for general purposes. 



Reference to the literature reveals a great number of formulae for 

 fixing solutions. These formulae are nearly all empirical, and not 

 based on any clearly defined knowledge of the utility of the various 

 ingredients. Of late years there has been a growing tendency to use 

 acetic acid in the preparation of these fluids, and the mode of action 

 of their different constituents has received careful attention. It 

 appears that acetic acid may be useful in two w r ays : first, by virtue 

 of its rapid penetration ; and, second, because it renders the proteid 

 substances in the tissues coagulable by other constituents of the fix- 

 ing solution — e.g., potassium bichromate, which, in neutral mixtures, 

 does not of itself coagulate these substances. If these conclusions be 

 correct, the addition of acetic acid to Muller's fluid should make it a 

 more valuable fixing solution, and its action would then be as follows : 

 the acetic acid penetrates rapidly and kills the cells ; it also renders 

 the proteids coagulable by the bichromate of potassium. The latter 

 salt penetrates but slowly, and at first acts in very slight concentra- 

 tion, not sufficient to cause gross coagulation, but nevertheless enters 

 into a combination with the proteids which, as the process goes on, 



1 A jar of absolute alcohol, prepared as above, may be used for the purposes of 

 fixing or hardening until the sulphate of copper has become of a deep-blue color, 

 or the alcohol so impregnated with dissolved fat that the latter interferes with 

 embedding in celloidin. When the latter is the case, the hardened celloidin is 

 opaque or opalescent. 



