248 THE RETINA, BY MAX SCHULTZE. 



a very apparent and regular transverse striation, and leads 

 quickly to a separation of the substance into disks. As the process 

 of imbibition in many instances does not occur with uniformity,, 

 curvatures, crook-like archings, and various other alterations of 

 form, are occasioned in the outer segment, the final result of 

 which is a spheroidal body resembling certain myelin drops. 



The larger rods of the fresh retina of the Frog, isolated in 

 serum, always exhibit in parts a very fine transverse striation 

 when examined with centric illumination and a magnifying 

 power of from 500 to 800 diameters. When this is not at first 

 observable, it can be rendered distinct by very oblique illumina- 

 tion* As soon as swell ing 'takes place in the substance of the 

 outer segment, in consequence of imbibition, disks may be seen 

 to separate, and these again undergo further change, especially 

 in serum diluted with water, swelling up and becoming ulti- 

 mately totally unrecognizable. The rods of the retina, both of 

 Man and animals, undergo precisely similar changes. The 

 highest powers of the microscope and very oblique illumina- 

 tions are however indispensable at an early period, before 

 swelling with prolongation of the external segment has taken 

 place. The external segment of the rods of Man and Mammals 

 which whilst still warm have been placed in a one or two per 

 cent, solution of perosmic acid, and have been thus preserved 

 absolutely unaltered in form, exhibit, when examined with a 

 power magnifying 1,000 diameters and very oblique illumina- 

 tion, a striation which is as sharp as a hair line drawn on 

 copper, and is about as fine as the markings of Nitschia sig- 

 moides, which is one of the most difficult test objects to resolve 

 amongst the Diatomaceae. This would correspond to a distance 

 between the lines of 0.3 to 0.4 of a micromillimeter. In the 

 cones the disks are somewhat thicker.f 



* Max Schultze, Archiv fur Mikroskop. Anatomic, Band v., p. 380. 

 Note. 



t Direct measurements may be found in Schultze's paper in the Archiv 

 fur Mikroskop. Anatomic, Band iii., p. 228, and in that of Zenker, idem, 

 p. 259. By the application of a more perfect system of lenses, I now 

 obtain somewhat smaller numbers than those given in the above. W. 

 Krause's objections are contained in his Essay on the Membrana fenes- 

 trata, p. 23. 



