Monthly Miciosoopical l 
Journal, Nov. 1, 1SG9. J 
Histology of the Eye, 
245 
microscopic objects ; wliilst the appendages are pale, have a low 
refractive index, and are less evident. 
The inner and the outer segment are separated by a sharp trans- 
verse line, where the slightest violence snaps them asunder. 
The rod-shaft is a long, slender cylinder — in profile, a narrow 
rectangle. The ends are truncated ; the outer rests on the choroidal 
epithelium, and the inner joins the appendage. In the perfectly 
fresh shaft I cannot discern any differentiation of parts, except an 
external outline, indicative of a containing membrane, and a homo- 
geneous contained substance ; but very soon after death the shafts 
begin to alter. The fresh perceptible change is, I think, a very faint 
longitudinal striation, and this is followed by the appearance of cross 
lines, which divide the shaft into light and dark segments ; at the 
same time the shafts swell and bend and lose their rectilinear figure. 
This segmentation, which must have been familiar to every one 
since Hannover first wrote on the retina, I have never seen in abso- 
lutely fresh shafts examined instantly after death; so that, in 
common with others, regarding it as a jpost mortem change, I did 
not attach much importance to it. Professor Schultze, however, 
has founded upon it the ingenious view that the shafts are built up 
of discs of alternately nervous and connective substances. 
The inner segment or rod-appendage has commonly the shape 
of a slender triangle or spindle ; and one of the outer granules, as I 
shall shortly show, is always associated with its inner end. In its 
outer end, immediately inside the line which marks it off from the 
shaft, there may often be seen, particularly in the large rods of 
amphibia, a small hemispherical body of the same refractive index 
as the shaft to which it sometimes remains attached when the shaft 
and appendage separate. It was long ago described by the late 
H. Miiller, whose loss every histologist deplores, and I figured it 
myself in a communication to the Eoyal Society in 1862. Schultze, 
who has lately called attention to it, suggests that it may act as a 
collecting lens. 
The outer segment of the cones — the cone-shaft — is usually 
shorter than the rod-shaft, and it commonly tapers shghtly outwards, 
the outward end being slightly narrower than the inner. The cone 
appendage is usually flask-shaped or bulbous ; and, like the corre- 
sponding part of the rod, its inner end always has its associated 
outer granule." In the outer end of the appendage in birds, in 
some reptiles, and in batrachians, lies the well-known coloured bead 
which forms so exquisitely beautiful a microscopic object in the 
retina of these animals. 
The interstices between the bacillary elements are occupied by 
a soft, homogeneous connective substance, which in all vertebrates 
below mammals contains a granular pigment. This extends inwards 
from the choroidal epithelium around and between the shafts as 
