XIII.] THE FROG. 249 



h. The retina. 



1. Sufficiently satisfactory specimens of this organ can 

 be obtained as follows. Take perfectly fresh eyes from 

 a frog, prick the corneas in two or three places, and 

 lay the eyes aside for three or four days in 0*25 g 

 chromic acid solution : then transfer them to alcohol 

 and keep them in it until wanted. 



a. Carefully cut open an eye preserved in the 

 above method and expose the retina : trans- 

 fer the latter to a glass slide, and with a razor 

 chop down on it so as to cut off a number of 

 slices : add glycerine, put on a cover, and examine 

 with a low power. Some of the bits will be found 

 thin enough for further examination. 



1). With the low power little can be seen but that 

 the retina is composed of a number of different 

 layers, some of which appear less opaque than the 

 others. 



c. Put on a high power and examine, make out the 

 following points 



a. The internal limiting membrane, a thin struc- 

 tureless layer. 



0. The nerve-fibre layer : thin and granular. 



[Both a and fi are often difficult to make out in 

 retinas prepared as above.] 



7. The nerve-cell layer : composed mainly of cells 

 like those described above (g. 1. a), but rather 

 smaller than those from the sympathetic ganglia. 

 From some, branches can be traced into the 

 next layer. , , . > 



