NUNNELKY, ON THE RETINA. 235 
separates them from the rods, and they are, as it were, im- 
bedded in the vesicular layer, which, without care, always 
renders the fibres indistinct. These may be seen in man, 
animals, birds, reptiles, and fish, on both sides of the fibres. 
(See Pls. X and XI.) 
Much discrepancy of opinion prevails as to the exact 
nature of these fibres. Hassall and Bowman assert that 
they are flattened solid fibres of gray matter, not tubular, 
the white nerve-matter ceasing as the nerve passes through 
the sclerotic coat, or almost immediately after it has entered 
the eye, and that in passing forwards they anastomose, so 
as to form elongated meshes, in which nucleated vesicles 
appear. Hannover states that the fibres in the retina appear 
to possess greater consistency than they do in the trunk of 
the nerve itself, but otherwise have the same character. That 
they proceed forwards in straight lines, without subdividing 
or anastomosing, and never become varicose. (The latter 
statement he modifies in a note, and allows that they do, 
when not in a natural state, become varicose.) Kolliker 
says, each fibre enters the eye without its sheath of con- 
nective tissue, independent of the others ; that, radiating in 
all directions, they constitute a continuous membrane as far 
forwards as the ora serrata, running parallel to each other, 
or inosculating at very acute angles, are without nuclei, and 
form frequent varicosities. He styles them the horizontal 
fibre system, in contradistinction to the rods and cones, 
which he calls the radiating fibre system. My own observa- 
tions agree more nearly with those of Hannover and Kolliker, 
than with Hassall and Bowman, though not exactly with 
either. If the formation of varicosities is to be regarded as 
showing the tubular character of nerve fibres, I can enter- 
tain no doubt that those of the retina are as much tubes as 
those in the optic nerve, with which I believe they are directly 
continuous, and also possess the same characters. I have 
seen the fibres in the retina in man, sheep, oxen, birds, rep- 
tiles, and fish become varicose while under examination ; 
the addition of a little water, dilute acetic acid, or mere 
variation in the pressure, will make them so at once, and often 
after being in weak spirit for a short time they are found so, 
particularly when the cerebral cells are gently removed 
before examination. When a particle of the optic nerve, and 
another of the retina, of perfectly fresh eyes of almost any 
mammal or fish, are examined with the assistance of either 
very dilute ammonia or chromic acid, it would be very diffi- 
cult to point out the difference in the fibres of the two; 
their size is the same; and with those from the retina, per- 
fectly clear cells with double walls, different from the nu- 
