FIXATION AND HARDENING. 305 



CHAPTER XXIX. 



RETINA, INNER EAR, NERVES. 



Retina. 



644. Fixation and Hardening. For section cutting, the 

 retiua of small eyes is best prepared by fixing the entire un- 

 opened bulb with osmium vapour. According to RANVIER 

 (Traits, p. 954) you may fix the eye of a triton (without 

 having previously opened the bulb) by exposing it for ten 

 minutes to vapour of osmium. The sclerotic being very thin 

 in this animal, such a duration of exposure is generally suffi- 

 cient. Then divide it by an equatorial incision and put the 

 posterior pole for a few hours into ^-rd alcohol. Stain for some 

 hours in picro-carmine (1*100), treat again with osmic acid 

 " so as to definitively fix the elements," wash with water, and 

 harden in alcohol. 



Somewhat larger eyes, such as those of the sheep and calf, 

 may be fixed in solutions without being opened. But it is 

 generally the better practice to make an equatorial incision, 

 and free the posterior hemisphere before putting it into the 

 liquid. 



Tlio older practice was to use strong solutions of pure osmic 

 acid ; but most of the best recent work has been done with 

 chromic mixtures. 



BARRETT (Quart. Journ. Mic. Sci., 1886, p. 607) recommends 

 fixing the entire unopened bulb in a liquid containing 0*2 per 

 rent, of osmium and -^th per cent, of chromic acid in water, 

 for twenty-four to thirty-six hours, with subsequent harden- 

 ing for fourteen days in alcohol containing 2 per cent, of 

 carbolic acid. He finds that the preservation of specimens so 

 treated i> lar superior to that of specimens hardened in pure 

 alcohol. 



SCHIEFFERDECKEK (J.?y//. /. mlk. Aunt., 1886, p. 305) used 



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