GENERAL TECHNIC 5 



(b) Formaldehyde, in very dilute solutions (0.2- to 0.4-per-cent. 

 commercial formalin^). — Tissues should remain in the formaldehyde 

 solution from twenty-four to forty-eight hours. This is also espe- 

 cially useful for dissociating epithelial cells. 



(c) Sodium or Potassium Hydrate (30- to 35-per-cent. aqueous solu- 

 tion). — From twenty minutes to an hour is usually sufficient to cause 

 the tissue elements to fall apart or to be readily pulled apart with teas- 

 ing needles. If it is at any time desirable to stop the action of the 

 caustic alkali, this may be accomplished by neutralizing with glacial 

 acetic acid or by replacing the alkali with a 60-per-cent. aqueous 

 solution of potassium acetate. The specimens may then be preserved 

 in the potassium-acetate solution, in glycerin, or in 50-per-cent. 

 alcohol. This dissociating fluid is largely used for muscle cells and 

 fibres. 



(d) Nitric acid (10- to 20-per-cent. aqueous solution). — This is 

 especially useful for dissociating involuntary and voluntary muscle. 



After any of the above procedures, the macerating fluid containing 

 the tissue elements should be placed in a long tube, allowed to stand 

 for a time and the fluid decanted. Water is then poured into the 

 tube, the tissues allowed to settle and the water poured ofl", this being 

 repeated until all trace of macerating fluid is removed. This washing 

 is facilitated by using the centrifuge. The tissue elements may then 

 be preserved or mounted in glycerin or in glycerin jelly. It is fre- 

 quently advisable to stain the tissues. For this purpose alum-car- 

 min (p. 19), and picro-carmine (p. 21), are especially satisfactory. 

 (For details see technic i, p. 129 and technic 2, p. 129.) After stain- 

 ing and washing, the tissues may be preserved or mounted in glycerin, 

 eosin-glycerin, or glycerin jelly. It frequently happens that on 

 examining dissociated tissue elements after mounting, the bits of 

 tissue are still too large. This may be remedied by gently tapping 

 on the cover-glass with a lead pencil. 



PREPARATION OF SECTIONS 

 I. Fixation 



Even tissues, the structure of which admits of their being exam- 

 ined in the fresh condition, as described on p. 3, soon undergo 

 post-mortem changes if placed in an inert medium such as normal 



' Commercial formalin is a 40-per-cent. solution of formaldehyde gas in water. 



