264 PRACTICAL HISTOLOGY. 



an eye that has been in Miiller's fluid or bichromate 

 of potash (2 per cent.), and subsequently in spirit. 

 A piece is cut out as before, and is treated in a simi- 

 lar way, except that before being stained the pigment 

 is brushed completely off the posterior surface with 

 a stiff camel-hair brush. This must be done under 

 spirit, and of course very carefully so as to avoid 

 tearing the tissue; during the operation the iris is 

 examined now and then with a low power, to deter- 

 mine when all the pigment is removed. It is diffi- 

 cult to prevent some of the pigment granules from 

 still sticking to the surface, but, as they tend for the 

 most part to adhere along the lines of junction of 

 the fibre cells, their presence does not spoil the object 

 of the preparation, for the dilatator fibres are if any- 

 thing better displayed. 



THE RETINA. 



Preparations 21 and 22. Sections of the 

 retina. It will be well to study the general ar- 

 rangement of the several layers of the retina by means 

 of sections, before its constituent elements are ob- 

 served isolated. Two methods of hardening may be 

 recommended, viz. by Miiller's fluid and by osmic 

 acid, the process being in both instances completed 

 by spirit. If possible to obtain it perfectly fresh and 

 healthy, the human retina should always be used ; 

 if not so obtainable, that of the pig is preferable to 

 the retina of most other animals, and in the follow- 

 ing preparations it will be assumed that the eyes of 

 that animal are employed. 



To harden the retina in Miiller's fluid it is better 

 to keep the bulb whole, so that the membrane re- 

 mains supported by the vitreous humor. But to let 

 the fluid in readily two or three cuts must be made 

 in the sclerotic with a razor, or a piece even be cut 

 out at one or two places. The eye is then dropped 

 into a relatively large quantity of Miiller's fluid, and 

 allowed to remain in it for a week, the fluid being 



