THE RETINA. 265 



stirred at intervals ; the globe is then cut across, and 

 left for another week in the liquid, after which time 

 it is transferred first for twenty-four hours to weak, 

 and then to strong spirit. In another day or two 

 it is ready for the preparation of sections. Small 

 pieces from different regions may be taken, but as 

 they are all treated in the same way, they may be 

 described as one: 



The staining of the tissue is first effected by 

 placing the piece for twenty-four hours in alcoholic 

 logwood (Kleinenberg's). From this it is transferred 

 to spirit, then embedded in wax-mass. It is better 

 so to place it in the embedding tray that the sections 

 shall be both vertical and meridional ; since, made 

 in this way, they will take the general course of the 

 fibres of the optic nerve. The sections cannot be 

 made too thin, but they should be complete, that is 

 to say, they should include all the layers of the 

 membrane. One or two of the thinnest are to be 

 mounted in glycerine ; the others may be transferred 

 from the spirit to oil of cloves, and mounted in 

 dammar. In these preparations the nerve fibres, 

 the Mullerian fibres, the inner and outer granules, 

 and in fact the layers generally, are all well dis- 

 played, but the rods arid cones are much altered, and 

 the natural form and appearance .of the other elements 

 is by no means well preserved. 



In this respect the osmic method yields far better 

 results. The bulb in this case is cut open, and either 

 the whole or a piece only of the retina is taken arid 

 placed in 2 per cent, solution of osrnic acid. Here 

 it is left for several hours, and then, after being an 

 hour or two in water to get rid of the excess of 

 osmic acid, it is transferred to alcoholic logwood for 

 twenty -four hours. From this it is placed in strong 

 spirit, where it may be left until wanted. It is well 

 to embed the piece by the cacao-butter process, since 

 by hardly any other method can sufficiently thin 

 and complete sections of a retina that has been 

 hardened in osmic acid be obtained. For this pur- 

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