system ( 199, 201, 204) are; (IQ) Muller's fluid, (20) Erlicki's fluid 

 (21) Potassium dichromate (in aqueous solution). 



In the combination of chemicals in fixing solutions such as those 

 mentioned above, they should be chosen to supplement each other's 

 actions as far a> po>>ible and correct or counteract their defects. The 

 combinations must be chosen always with a view to the result desired 

 and frequently the components and their relative amounts determined 

 empirically, by experiment . 



7. The following general rules should be regarded in the 

 fixation of tissues niid organs: 



(1). The volume of the fixing fluid used should be large, ex- 

 ceeding the volume of the tissue at least thirty times. The less 

 energetic the action of the fixer the greater the amount of fluid to be 

 employed. When the fluid becomes turbid it should be changed to 

 fresh at once. 



(2). Fix only as small pieces of tissue as possible, or as is prac- 

 ticable from the results desired. The block of tissue should not 

 be more than 1 cm. in one dimension, and if possible let it be much 

 shorter, only 2 to ,5 mm. In some cases (Flemming's and Hermann's 

 fluids) much smaller masses are needed (1 to 2 mm. thick). This is 

 desirable for the rapid and complete penetration of the fixer. Of 

 course, in the case of entire organs it may not be possible to comply 

 with the conditions. Fixation by injection may then be resorted to. 

 Physiological salt solution ( 35) is first injected through artery of the 

 organ or part in order to remove the blood, and this is followed by 

 the injection of the fixing fluid. 



In addition to these two general principles, there are four points 

 to be carefully considered, upon which the excellence in the results 

 attained depends; they are (a) the fixer chosen, (b) the time of 

 fixation, (c) the trashing out of the fixer, (d) the hardening in alcohol 

 and the subsequent treatment. 



(a) The choice of a fluid into which the tissue is placed should 

 be made dependent on (1) a consideration of the particular feature 

 whose preservation is desired and the degree of excellence of fixation 

 that is desired or necessary, whether detail of cell structure or the 

 structure of the tissue in terms of cells or structural elements be 

 sought ; ("2 ) the penetrating power and the size of the piece that it is 



