THE RETINA. 267 



readily alterable elements, such as the rods and 

 cones and their nuclei, in a condition as nearly as 

 possible approaching that which they possess during 

 life. 



Other portions of fresh retina are to be placed, one 

 in one-eighth per cent, of bichromate of potash solu- 

 tion (for a week), and the other in ten per cent, of 

 chloral hydrate solution (for two or three days). The 

 portions so macerated are to be teased out in a drop 

 of their respective solutions, the usual expedient 

 beinoc adopted of obviating the pressure of the cover- 

 glass" by a hair. In the one treated with bichromate 

 the Miillerian fibres, and in fortunate preparations 

 the ganglion cells with their processes, may be well 

 seen'and isolated. The granules of the inner nuclear 

 layer are also frequently obtained with one or both 

 processes extending from their opposite poles, but the 

 rods and cones are for the most part much altered 

 and granular in appearance, so as hardly to be recog- 

 nizable. 



In the chloral hydrate preparation, the last-named 

 elements are much better preserved. The external 

 segments of the rods, which even in the osmic pre- 

 paration tend for the most part to become altered, 

 may by this method be frequently seen almost 

 unchanged, except that the transverse striation, 

 which indicates their discoid formation, is often 

 well marked. Most of the other structures are also 

 well preserved, and the Miillerian fibres and ganglion 

 cells are sometimes better seen than in the bichro- 

 mate preparation. Unfortunately neither the bichro- 

 mate nor the chloral specimens can be well preserved 

 without deterioration. 



Preparations 26 and 27. The study of the 

 retina cannot be considered complete until the ele- 

 ments have been examined in the fresh, unaltered 

 condition. A small piece, taken from an eye still 

 warm from the animal, should accordingly be broken 

 up as rapidly and finely as possible in a drop of 

 serum, or, failing this, in a little vitreous humor. 



