NEUROLOGICAL METHODS. 431 



Retina.* 



779. 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 RANVIUR 

 (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 

 sufficient. 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. 



Dr. Lindsay Johnson tells me that he now gets the best 

 results by fixing the globe over the steam of a 1 per cent, 

 osmic acid solution raised to the temperature at which vapour 

 is seen to be given off (but not to boiling-point), for five 

 minutes in the case of human adults, or for one to three 

 minutes in the case of human infants, all monkeys and small 

 mammals, as in them the sclerotics are very thin. As soon 

 as the sclerotic is felt to be firm to the touch, it should be 

 opened by a small nick with a razor just behind the ciliary 

 body ; or if the eye be that of an adult, the cornea and lens 

 may be removed. The eye is then put for twelve hours into 

 the mixture, 49 ; it is then washed in running water, and 

 suspended in a large volume of 2 '5 per cent, bichromate of 

 potash for two days, then passed gradually through successive 

 alcohols, beginning with 20 per cent., and ending with 

 absolute, taking five days from first to last. 



Other hardening liquids, however, also give good results, 

 provided that the fixation by the osmic acid has been 



* Besides the sources quoted in the text, see on this subject SELIGMANN, 

 Die mikroskopischen Untersuchungsmet hoden des Auges, Berlin, S. Karger 

 (Karlstrasse 13), 1899, pp. iv, 248. 



