266 PRACTICAL HISTOLOGY. 



pose it is as usual first placed in oil of cloves, and 

 then allowed to soak for half an hour or more in 

 cacao-butter, which is kept just melted by a gentle 

 heat. When completely permeated the piece of 

 membrane is placed in the proper position in a 

 cavity scooped out in a cake of cacao-butter, and 

 after it has firmly set, sections mny without diffi- 

 culty be made of exquisite fineness. They are placed 

 in oil of cloves in winter it must be very slightly 

 warmed which speedily dissolves out the infiltrated 

 cacao-butter, and they may then be mounted in 

 dammar. But it is better to transfer them from 

 the oil of cloves to spirit again, and afterwards to 

 mount them in glycerine, since in this the details 

 can be made out with greater readiness than in the 

 more highly refracting dammar. For the same 

 reason a saturated solution of acetate of potash is 

 sometimes recommended as superior to glycerine 

 for mounting osraic preparations, especially of the 

 retina. 



Preparations 23, 24, and 25. Isolation of the 

 retinal elements. Various processes are employed 

 for macerating portions of the retina in order to 

 obtain its elements, either in a completely isolated 

 condition or still partially connected with one 

 another. It will be best, in the first place, to at- 

 tempt the separation with a piece of retina which 

 has been, in 2 per cent, osmic acid for six or eight 

 hours. It must of course be put in perfectly fresh, 

 and after the time mentioned it is placed in water 

 for twenty-four hours. It is then allowed to mace- 

 rate for a few days in a mixture of glycerine, alco- 

 hol, and water (glycerine one part, alcohol one part, 

 water two parts), after which a minute portion is 

 to be carefully broken up with fine needles in a 

 drop of weak glycerine, and, a piece of hair having 

 been added, the cover-glass is superposed, and the 

 preparation examined with a dry objective and, 

 after fixing the cover-glass, with an immersion. 

 The effect of the osmic acid is to preserve the more 



