88 



three are not as strictly fixers as hardeners. The following solu- 

 tions containing these salts, combined or uncombined, are used as 

 fixers in this course : (i) Osmic acid, aqueous solution ; (2) Her- 

 mann's fluid ; (3) Flemming's fluid (stronger formula) ; (4) Picric 

 alcohol ; (5) Mercuric chlorid, in aqueous solutions ; (6) Picro- 

 aceto-sublimate (vom Rath's fluid) ; (7) Zenker's fluid ; (8) Miil- 

 ler's fluid; (9) Erlicki's fluid; (10) Potassium dichromate in 

 aqueous solutions; (11) Chrome-oxalic; (12) Alcohol (absolute — 

 67% strength) ; (13) Formaldedyde ; also, as embryological fixers, 

 (14) Perenyi's fluid ; (15) Picro-sulphuric ; (16) Picro-nitric ; (17) 

 Nitric acid. 



§ 19. The following general rules should be regarded in the 

 fixation of tissues and organs : 



(i) The volume of fixing-fluid used should be large, exceed- 

 ing the volume of the tissue at least thirty times. The less ener- 

 getic 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 practi- 

 cable with the results in view. The block of tissue should not be 

 more than i cm. in any diameter, and, if possible, let one of the 

 diameters be much shorter, — only one-fourth or one-half of a centi- 

 meter. This is desirable for the rapid and complete penetration of 

 the fixer. Of course, in the case of entire organs, it may not be pos- 

 sible to comply with the conditions. 



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 Jixer chosen, (^) the itme of fix- 

 ation, (f) the washfing out of the fixer, and {d ) the subsequent hard- 

 ening in alcohol. 



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

 should be made dependent on ( i ) a consideration of the degree of 

 excellence of fixation that is desired or necessary, — whether, in other 

 words, details of cell structure or the structure of the tissue in terms 

 of cells, be sought ; (2) the penetrating power of the fixer and the 

 size of the piece of tissue that it is necessary to have ; and (3) the 

 stain that is desired subsequently, which is largely affected by the 

 fixer employed. For example, Hermann's fluid is possibly the finest 



