The disc appears particularly well-defined, owing to the contrast of colour between it and the choroid.
The abundance of colouring matter in the pigment cells and granules of the choroid gives the latter the dark bluish brown colour.
The little light which reaches the sclerotic, and returns from it across the strongly-pigmented choroid, causes the blood in the vessels in the retina to appear dull red; while in the optic disc the blood has a brilliant red colour through the light returning from the white and opaque con- nective tissue of the optic nerve. The contrast in colour between the arteries and veins of the retina is well marked.
Fig. 4.
The region of the yellow spot, with the yellow spot in the centre, of an eye with blue iris (from a person aged 20).
The bluish rim represents the iris under the influence of atropia.
In the middle of the red surface of the figure is a yellowish white ring, which represents the image of the metal frame of the ophthalmoscope used in the examination.
This image is circular only round the yellow spot. It is not represented in the yellow spot of the other eyes.
In the centre of the ring is the yellow spot itself, which appears as a minute red dot, shading off into the surrounding parts.
Fig. 5.
The region of the yellow spot and the yellow spot itself, of a person, aged 20, with brown irides.
The brownish rim represents the iris; in the centre of the brown-red surface we observe a grey-brown ill-defined spot; this is "the yellow spot."
'Fig. 6.
The region of the yellow spot, and the yellow spot itself, of a person, aged 20, with black irides.
The brownish-black rim represents the iris; in the centre of the surface surrounded by it we see a black dot—"the yellow spot." The
