The organs of vision : their structure and functions / by G.J. Witkowski ; tr. by Henry Power.

  • Gustave-Joseph Witkowski
Date:
[1878?]
Catalogue details

Licence: Public Domain Mark

Credit: The organs of vision : their structure and functions / by G.J. Witkowski ; tr. by Henry Power. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    posterior lamina is reflected with the ligamentum pectinatum upon tlie iris, and has received the name of the membrane of Descemet, <>r of Desmoiirs. Man and the greater number of animals have only one cornea to each eye. Insects, on the contrary, which possess multiple vision, have as many corneae as thej^ possess eyes. In the cock- chafer, for example, there are 8,820, whilst the butterfly has :U,G50. 3. Choroid.—The name of this tunic is derived from its resem- blance to the deep layer of the skin which is termed the chorion. It is composed of three layers of different texture. The superficial is areolar (134); the deep, pigmentary (139); and the interme- diate, vascular (136). i. The areolar layer, of brown colour and woolly aspect, pre- sents whorl-like strise, which correspond to the flexures of the veins of the subjacent vascular layers. ii. The middle, or vascular layer, is composed of veins, arteries, and capillaries disposed in concentric layers. The veins, which are superficial, are named the vence vorticosce (112), because their rootlets describe parallel curves like a whorl; they unite to form four principal trunks. The capillaries, which form the inner- most layer, are, like the veins, arranged in a stellate manner, with curved rays. This layer of capillaries, known as the chorio- cainllaris (138), gives to the eye the deep red tint seen on ophthal- moscopic examination. The arterial ramifications (137) proceed from the short posterior ciliary arteries (114), and occupy the space between the two other vascular planes. iii. The pigmentary layer, deeply placed, is very dark in tint, and owes its colour to the presence of a great number of pigment granules similar to those found in the skin of the negro. It serves to absorb the luminous rays which have acted on the retina, and which are no longer useful for vision. Were this layer absent, the light entering the eye would be reflected in all directions, and would render the retinal image indistinct. This is actually ob- served in Albinos, in whom the choroid is destitute of pigment- granules. In old age the pigment-cells undergo fatty degeneration, and assume a grey colour, which renders the tint of the fundus of the eye of the aged less deep than that of the adult. This natural change is often mistaken by the inexperienced for incipient cata- ract. [In certain animals the internal surface of the choroid presents a beautiful iridescent appearance, or metallic lustre, named tapetum, which is due to a peculiar arrangement of the texture of this membrane.]