THE EYE. 371 



rods," and, although they had often seen the somewhat longer 

 filaments attached to them, the latter were regarded merely as 

 artificial products. Since Hannover, also, the points of these 

 organs have been misplaced outwardly by all writers, which is 

 wholly incorrect. The substance of the "rods" is clear, 

 homogeneous with a faint glistening fatty aspect, very soft and 

 flexible, and at the same time extremely fragile. Their deli- 

 cacy is so great that they undergo the most manifold changes 

 even in water, often even to their being rendered unrecog- 

 nisable, bending, as it were, into a hook of various forms, 

 curling and rolling up in all ways, or breaking up into two 

 or more pieces, and allowing clear drops to escape, which 

 are often met with on the external surface of the retina 

 in vast quantity, derived partly from the " rods," partly from 

 the ruptured pigment-cells of the choroid. One of the most 

 usual changes consists in this, that the point, if it be not 

 detached, which is very frequently the case, becomes distended 

 in a varicose manner, and assumes a lancet-shape, or is even 

 transformed into a sphere, on which the " filament" of various 

 lengths is placed, in consequence of which, the obtuse end of 

 the M rod" often presents a hook-like curve or a slight enlarge- 

 ment. The "rods" are almost invariably very much altered 

 by reagents ; and, above all, the proper " rods," which, not- 

 withstanding their greater bulk, yet offer less resistance than 

 the "filaments." Ether and alcohol cause them to contract 

 and shrivel up, often rendering them unrecognisable, but do 

 not dissolve them. In acetic acid of 10 per cent, they are 

 immediately shortened very considerably, swelling out in 

 several places, and disintegrating into clear drops, which at 

 first offer some resistance, but afterwards disappear. Concen- 

 trated acetic acid dissolves them in a short time, as do alkalies 

 and mineral acids; whilst diluted chromic acid, although it 

 causes them to shrink a little, is the best preservative of 

 them. 



The u cones 9 ' (fig. 303 4 ) are " rods" which instead of a 

 filament are furnished at their inner extremity with a conical 

 or pyriform body, the length of which equals half the thickness 

 of the bacillar layer (from 0007 — O'Olo"'), and whose breadth 

 is from 00025'" to 0-0045'". Each of these "cones" consists 

 of an external, thicker and longer, finely granular extremity, 



