Page:Plates illustrating the natural and morbid changes of the human eye.djvu/21

Rh pigment (an accumulation of pigment granules level with the hexagonal cells); this is sometimes observed in healthy eyes. The middle of the optic disc has a whitish, the rest a brilliant pink colour (the disc in the figure is too red). The whitish portion is solely occupied by opaque white connective tissue, which surrounds the blood-vessels where they pass through the optic disc. The bundles of optic nerve fibres occupy the pink portion of the disc.

All the blood-vessels of the retina pass in and out through the optic disc, and from it diverge into the retina (the larger number branch towards the left side in the direction of the region of the yellow spot). The brilliant red colour, which surrounds the optic disc, is caused by the light of the ophthalmoscope (reflected from the white inner surface of the sclerotic), having to pass through the transparent blood-carrying vessels of the choroid.

Fig. 2.

The same parts as represented in Eig. 1, from a hypermetropic person aged 20, with brown irides.

The optic disc (in the middle of the figure) is oval- shaped; a peculiarity which is often observed in hypermetropic persons with convergent strabismus.

The numerous blood-vessels of the retina diverge from the optic disc. The veins are somewhat tortuous, as is often seen in young asthenopic hypermetropics.

Many of the vessels appear bulged forwards towards the vitreous chamber, at the spot where they pass from the optic disc upon the retina; this appearance is peculiar to hypermetropics.

The grey red colour of the rest of the figure is caused by the light (reflected from the sclerotic) having to pass through the more highly-pigmented vascular choroid.

{{center|Fig. 3.

The same parts as represented in Fig. 1, of a hypermetropic negro, aged 28, with " black irides."

The centre of the oval-shaped optic disc has a brilliant white, the rest a delicate pink colour. {{nop}}