FOVEA CENTRALIS OF THE HUMAN RETINA. 
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fibres agree with that which obtains in other parts of the retina, only their branches are 
with difficulty traceable through the granular band lying between the cone-fibre plexus 
and the inner granule-layer, in which most of them appear to end. They are also struc- 
turally connected with the finely areolated tissue composing the granular layer, and, 
further, with the interstitial tissue which pervades all the layers. 
Blood- Vessels . — In none of my sections have I found blood-vessels at the centre of the 
fovea ; at the margin, however, capillaries occur, and small arteries are not uncommon 
within the limits of the macula. The vessels nowhere penetrate beyond the outer 
surface of the inner granule-layer. 
Deductions. 
1. Since the total of the effects of light upon living tissue will be greater as the 
extent of tissue traversed by it is greater, and since the relative common sensitiveness of 
a surface varies with the number of distinct sentient elements it contains, it follows 
that the greater length of the cones and rods, and their greater slenderness, which allows 
a larger number of them to the superficial unit, are in harmony with the greater sensi- 
tiveness of the retina at the macula lutea. Inasmuch, however, as the foveal cones are 
stouter than the rods, a superficial unit at the centre of the fovea contains fewer sentient 
( i . e. percipient) elements than the same unit near the periphery of the macula lutea ; and 
on this ground the sensitiveness of the retina at the fovea should be less than that of the 
retina near the periphery of the macula. On the other hand, the extreme thinness of 
the inner layers of the retina at the centre of the fovea, places the bacillary layer here 
most favourably for receiving incident light. 
2. The division of the rods and cones into an outer and an inner segment is natural. 
The facts in support of this are, the presence of the division in perfectly fresh specimens ; 
its sharpness and constant occurrence at a definite place ; the constantly rectilinear 
figure of the outer, and the curvilinear figure of the inner segment ; the different refrac- 
tive powers of the segments ; and their different behaviour towards staining and chemical 
solutions. 
3. From these structural differences it is a fair inference that the segments have dif- 
ferent physiological meanings. 
The higher refractive power, straight sides, and slender cylindrical or prismatic figure 
of the outer segment may be adaptations for confining within the segment light incident 
upon its end, and for preventing the lateral escape of light through the sides of the 
segment into neighbouring cones and rods. These considerations incline me to adopt 
the opinion that this segment has an optical function, an opinion which derives further 
support from the fact that, in those animals in which the segment is so wide a cylinder 
that a ray might be incident upon the inner surface of its sides at a small enough angle 
not to be reflected but to pass out, the segment is insulated by a sheath of black 
pigment. 
The inner segments of the cones and rods are the specially modified peripheral termi- 
