xxxii Journal of Comparative Neurology. 



am satisfied, however, that both the picro acetic formalin and the picro- 

 iron-alum-formahn are very useful formulae for general histological pur- 

 poses, particularly where a slight decalcifying power is required. But 

 the meduUated nerves seem to be the tissues for which these mixtures 

 are least adapted, and for the purposes of this research all picric acid 

 combinations were discarded. 



III. — FIXATION IN CHROMIC ACID MIXTURES. 



Summary. — Chromic acid, hke nitric and picric acids, has a very 

 injurious effect upon the nerve sheaths, even when applied in very 

 dilute solutions and for a time far too short to permit of much decalci- 

 fying effect. Even in the presence of very strong formahn this injuri- 

 ous effect is still manifest. It is, moreover, in fishes, at least, not fa- 

 vorable for subsequent Weigert's staining. 



IV. — FIXATION IN ACETIC ACID MIXTURES, 



Summary. — Acetic acid alone, like chromic acid, has a deleterious 

 effect upon the nerve sheaths ; and this effect is exerted even in the 

 presence of formalin solutions sufficiently strong to fix the sheaths well 

 if acting alone. Acetic acid in combination with sublimate has a still 

 worse effect upon the medullated nerves, the white matter of the brain 

 being more or less gelatinized. Other combinations of sublimate which 

 I have tried lead me to condemn it (whatever may be its virtues for 

 other purposes) as a poor fixer for nerve fibers. 



v. — FIXATION IN CHROM-ACETIC MIXTURES. 



I have found Fol's chrom-acetic a most excellent fixing fluid, not 

 only for general tissues, but especially for the brains of bony fishes. 

 The medullary sheaths of the nerves are, however, less faithfully fixed 

 than most of the other tissues. The addition of formalin to the mix- 

 ture corrects this defect in a measure ; the fluid has not, however, suf- 

 ficient decalcifying power for the present purpose. The substitution of 

 formalin for the osmic acid in Flemming's stronger formula gave much 

 better results, as shown by the examples below. 



24 F. A small specimen (4 cm. long) was treated for seven days with 

 Chromic acid, i per cent., 15 parts. 



Glacial acetic acid, i " 



Formalin, 4 " 



The sections were mordanted in Wolter's fluid for 20 hours, 

 10 per cent, vanadium chloride, 2 parts. 

 8 per cent, aluminium acetate, 8 " 



They were stained for 24 hours in acid hematoxylin and differentiated 

 with Weigert's decolorizer. This gives very excellent preparations. The fixa- 



