THE MICROSCOPE IN ANIMAL HISTOLOGY. 221 



produced on the retina by the lens to be viewed by the 

 mind. The lens does, indeed, form an image on the mem- 

 brane, so it would on glass or paper, but the real action 

 of the vibrations of light upon the nervous conductors is 

 not thus to be explained. 



The complex structure of the retina is only recently 

 known, and it may be that many laws of light yet un- 

 known are to be exhibited by its means, as well as much 

 that relates to the connection of the perceiving thinking 

 mind and the external world. 



Muller's fluid, concentrated solution of oxalic acid, 0.6 

 per cent, solution of sulphuric acid, and 0.1 to 2 per cent, 

 solutions of osmic acid, may be used for hardening, but 

 very delicate dissection is required for demonstration. 

 Rutherford recommends chromic acid and spirit solution, 

 1 gramme of chromic acid in 20 c.c. of water, and 180 c.c. 

 of methylated spirit added slowly. 



The retina consists of the following layers : 1. The 

 columnar layer, or layer of rods and cones. 2. Membrana 

 limitans externa. 3. External granular layer. 4. Inter- 

 granular layer. 5. Internal granular layer. 6. Molecular 

 layer. 7. Ganglionic cell layer. 8. Expansion of optic 

 nerve. 9. Membrana limitans interna. To these may be 

 added : 10. The pigment layer, often described as the 

 pigmented epithelium of the choroid, into which the rods 

 and cones project. These layers are composed of two 

 different elements, mutually blended, a connective-tissue 

 framework of varying structure in the different layers, 

 and a complex nervous tissue of fibres, ganglia, rods, and 

 cones. Plate XXIV, Fig. 177, is a diagram of these 

 separate structures, after M. Shultze, in Strieker's Man- 

 ual of Histology. 



The structure of the rods and cones is complex, and 

 varies in different animals. The rods readily decompose, 

 becoming bent and separated into disks, but examination 

 of well-preserved specimens shows them to have a fibril- 



