358 RETINA, INNEE EAE, NERVES. 



CHAPTER XXIX. 



EETINA, INNEE EAE, NEEVES. 



Retina. 



682. Fixation and Hardening. For section cutting, the 

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

 opened bulb with osmium vapour. According to RANVIER 

 (Traite, 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 one-third 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. 



The older practice was to use strong solutions of pure osmic 

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

 chromic mixtures. 



KUHNT (Jen. Zeit.f.Naturw'., Bd. xxiv, H. 1, 1889, p. 177; 

 Zeit.f. wiss. Mik., vii, 1, 1890, p. 65) finds that for isolation 

 preparations the best way is to fix for twenty to twenty-eight 

 hours in f per cent, osmic acid, then put the preparation for 

 fourteen days into water in a well-closed vessel, changing the 

 water once at the middle of the time, and at the end of it bring- 

 ing the preparation for three to four weeks into a mixture 

 of 80 parts water, 12 parts alcohol, and 8 parts glycerin. 



For sections he prefers fixing for two days in solution of 

 Flemming, hardening for three days in alcohol, and imbedding 

 in celloidin. 



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

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



