246 
Histology of the Eye. 
[Monthly Microscopical 
Journal, JSTov. 1, 1869. 
far as their line of union with the appendages. It completely 
insulates the shafts, and would have the effect of absorbing any 
pencil of light which, making a relatively small incident angle, 
might escape laterally outwards through the shaft-wall, and in this 
way the escaped pencil would be prevented from entering a neigh- 
bouring shaft. 
In mammals, the greater slenderness of the shafts probably 
renders such a provision unnecessary, because the incident pencil, 
to enter the shaft, must nearly coincide with its axis ; and, as regards 
the side of the shaft, the angle of incidence would be so large that 
the pencil would probably be totally reflected. 
The inner ends of the rods and cones pass through apertures in 
the connective membrane, called the membrana limitans externa 
retinae, and are produced inwards amongst the outer granules as 
slender bands or fibres. The membrana limitans externa is the 
sharp, hard line, always perceptible in vertical sections between the 
bacillary and the outer granule layers. 
That the rods and cones are the percipient elements in the 
retina is now universally received, so that it needs hardly be men- 
tioned ; but it may be well to adduce the chief considerations on 
which this presumption rests. First, they alone of all the retinal 
tissues are so arranged as to be capable of receiving separate and 
distinct stimuli from small incident pencils of light. Next, their 
absence entails absence of perception. Mariotte's experiment proves 
this as regards the optic nerve disc, and the increase of the size of 
the blind spot in myopia from posterior staphyloma, proportionately 
to the extent of the white atrophic crescent — a fact which is easily 
roughly verified — is another proof of the same thing ; because here, 
together with the disappearance of the choroidal epithelium and 
chorio-capillaris, I have had opportunities of proving microscopi- 
cally the absence of the cones and rods. 
When we endeavour to press our inquiries farther, and try to 
ascertain what may be , the respective functions of the outer and 
the inner segment of the rods and cones, and in what respect the 
functions of the rods and cones diff'er, we meet with difficulties 
which have yet to be overcome. 
As regards the first part of this inquiry, the high refractive 
index of the shafts, and their insulation by a coat of pigment in 
many animals, points to a physical optical role; while the asso- 
ciation of a nucleus (an outer granule) with the appendage, suggests 
a more vital dynamical share. If this be so, then the junction 
between the shaft and appendage marks the line where, so to say, 
the physical vibrations of light are converted into nerve-force. 
Towards the solution of the second point of the inquiry, Schultze 
contributes the important fact that nocturnal mammals, as the 
mouse, bat, hedgehog, have no cones ; and that in owls, they want 
