PRACTICAL NOTES OX HISTOLOGY. 228 



away the superfluous stain in spirit. Next place the eye for a 

 short time in a strong solution of iodine-green and wash it well ; 

 soak in gum solution and freeze. The granular layers are stained 

 green, the others with rosein. Mount in Canada balsam, and do 

 not leave long in spirit. 



Osmic Acid may also be used as a hardening agent, finally 

 placing it in spirit. It is as well in many cases to stain the tissue 

 first (using logwood, etc.), transfer from the stain to spirit, then 

 embed in wax mass. It is best to place it in the tray in such a 

 way that the sections shall be both vertical and meridional, so 

 that we take in the general course of the fibres of the optic ner\-e. 

 Mount a few of the thinnest — they cannot be too thin — in glyce- 

 rine : the others transfer to clove oil and then balsam. In the 

 Osmic method cut open the bulb, and place the retina, or a part 

 of it, in a 2 per cent solution of the acid. Leave for several 

 hours, wash well in distilled water, and then transfer to alcoholic 

 logwood (Kleinenberg's). Place from it into strong spirit, where 

 it is left till wanted. They may be mounted, some in glycerine 

 and some in a saturated solution of acetate of potash. 



Isolation of the Retinal Elements.— Try, first, a piece taken 

 from a 2 per cent, osmic acid solution : wash well and allow to 

 macerate for a few days in a mixture of glycerine, alcohol, and 

 water (glycerine i part, alcohol i part, water 2 parts) ; after which 

 a minute portion is to be carefully broken up with fine needles in 

 a drop of weak glycerine, and then replace with pure glycerine. 

 Put other fresh portions of retina, one in one-eighth per cent, 

 bichromate of potash for a week, and another in 10 per cent, 

 chloral hydrate for two or three days. Tease out in their respec- 

 tive solutions, and avoid pressure by placing a hair under the 

 cover-glass. These preparations unfortunately will deteriorate. 



The retina ought also to be examined in a fresh, unaltered 

 condition. So take a small piece while warm and break up, 

 rapidly and finely as possible, in a drop of serum or vitreous 

 humour. The hexagonal pigment is seen in those hardened in 

 Miiller. The retina in the lower vertebrata and in fish, birds, etc., 

 should be teased out fresh in vitreous humour. 



An aqueous solution of aniline blue gives good results, as does 



