NERVOUS CONSTITUENTS OF THE RETINA. 24)7 



segment of the cones, which have been taken from the back 

 part of the eye, and preserved as perfectly as possible, to be 

 twelve micromillimeters. This is about one half of the length 

 of the corresponding part of the adjoining rods. Great differ- 

 ences in this respect exist amongst animals. Thus, for example, 

 in the Pig, the retina of which contains an extraordinarily large 

 number of cones, the small length of the latter in comparison 

 with the rods is very striking. At certain points the cones, 

 with their external segments, scarcely reach the line of junction 

 of the outer and inner segments of the rods. (See the adjoin- 

 ing woodcut.) 



The difference in the refractive powers of the two segments of 

 the rods and cones is apparent even in perfectly fresh specimens, 

 but becomes still more distinct after death, coincidently with the 

 occurrence of post-mortem changes that then rapidly take place, 

 even with the most careful treatment. These changes consist 

 partly in the originally homogeneous substance of the somewhat 

 less strongly refractile internal segment becoming cloudy and 

 finely granular, whilst the outer segment remains homogeneous 

 and highly refractile. As a consequence of this, the line of junc- 

 tion of the two segments is rendered more distinct. Whilst the 

 rods can be preserved for some time in indifferent fluids without 

 undergoing further alteration, a process of coagulation usually 

 occurs in the inner segments of the cones, rendering them 

 coarsely granular and progressively more and more opaque, their 

 outer segments soon becoming wholly unrecognizable. In these 

 last alterations set in almost unavoidably immediately after the 

 preparation of the fresh specimen, resulting in the arching and 

 curvation of the whole structure with division into disks, which 

 after remaining connected for some time ultimately become 

 swollen up and disappear. 



The same changes are undergone, though more slowly, by the 

 outer segments of the rods. The peculiar alterations that have 

 long been known to occur in them when floating in serous 

 fluids, especially after dilution with water, and which have 

 been regarded as a kind of coagulation, depend, as I have shown,* 

 upon imbibition of fluid, which in the first instance produces 



* Archivfiir Mikroskop. Anatomie, Band iii., p. 224. 



