268 PRACTICAL HISTOLOGY. 



A modification of this very simple method consists 

 in allowing a small piece to macerate for two or 

 three days in weakly iodized serum (see p. 45) before 

 attempting the dissociation, which can then be more 

 readily effected. 



Preparation 28. The hexagonal pigment of the 

 retina is seen in most of the teased preparations 

 above described. In eyes that have been hardened 

 in Muller's fluid the layer often separates in flakes 

 of varying size, and nothing is simpler than to re- 

 move such a piece with a section lifter, and mount 

 it in glycerine, so as to exhibit the pavemented 

 appearance which the cells present. 



Preparation 29. On the opposite surface of the 

 retina also a mosaic-like appearance can be demon- 

 strated by the aid of nitrate of silver, but it is much 

 more irregular, and does not depend upon the pre- 

 sence of epithelium cells, but upon the flattened-out 

 ends of the Miillerian fibres. To show this appear- 

 ance a fresh eye is cut in half transversely, and the 

 vitreous is shelled out from the posterior half; this 

 is then rinsed in distilled water and transferred to 

 nitrate of silver solution (one-half per cent.). After 

 a minute in this it is again rinsed in distilled water, 

 and exposed in water to the light. When the sur- 

 face is browned it is removed, a piece of the retina 

 is cut out under water, floated on to a slide with the 

 inner, brown surface uppermost, and the water is 

 drained off and replaced by glycerine, when the pre- 

 paration can be covered and examined, care being 

 taken not to let the cover-glass press upon the speci- 

 men. 



Preparation 30. The Retina in the lower 

 Vertebrata. The structure of the outer segments of 

 the rods, which it is difficult to make out in mammals, 

 can be seen easily, even with an ordinary high power, in 

 the retina of amphibia. With this object the eye is re- 

 moved from a recently killed frog, cut across, and a small 

 portion of the retina is quickly broken up in vitreous 

 humor. A piece of hair having been added, the prepara- 

 tion is covered and examined. Almost everywhere the 



