THE EYE. 741 



filament. The former portion of the rods, which alone almost has hitherto 

 been known to anatomists, is a cylinder, 0-0075-0-012-0-015 of a line 

 long, 0-0008 of a line broad, and truncated at the outer end, whilst the 

 inner is produced into a short point, 0-002-0-003 of a line in length, 

 which is often separated from the rest of the "rod" by a faint transverse 

 line, and might even perhaps be assigned to the " filament." The latter 

 is an extremely delicate process, not more than 0-0002-0-0003 of a line in 

 thickness, of uniform width throughout, prolonged immediately from the 

 point of the " rod, "and, extending through the inner half of the bacillar 

 layer ; it is connected with the other elements of the retina in a manner 

 to be afterwards described. The filament is so delicate that it is usually 

 torn off near its origin on the slightest mechanical impression affecting the 

 bacillar layer; on which account also it has happened that observers hi- 

 therto have been acquainted only with the " proper rods," and, though 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, homo- 

 geneous with a faint glistening fatty aspect, very soft and flexible, and 

 at the same time extremely fragile. Their delicacy is so great that they 

 undergo the most manifold changes even in water, often even to their 

 being rendered unrecognizable, bending, as it were, into a hook of vari- 

 ous 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 " rod" often presents a 

 hook-like curve or a slight enlargement. The "rods" are almost inva- 

 riably very much altered by reagents ; and, above all, the proper " rods," 

 which, notwithstanding their greater bulk, yet offer less resistance than 

 the "filaments." Ether and alcohol cause them to contract and shrivel up, 

 often rendering them unrecognizable, 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. Concentrated 

 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 "cones" (Fig. 303 4 ) are "rods" which instead of a filament are 

 furnished at their inner extremity with a conical or pyriform body, the 



