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Note: This text has been automatically extracted via Optical Character Recognition (OCR) software. The text has not been manually corrected and should not be relied on to be an accurate representation of the item.
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called the pupil , is that which allows the passage of the rays of light into the interior of the eye . This aperture varies in its dimensions , according to the quantity of light to which the organ is exposed : a strong light causes
the pupil to become contracted , to exclude a portion of rays of light which would offend the organ . In weak light v •* aperture is enlarged to admit as mai rays as possible . The name of iris was originally applied to this part , from the diversity of colours observable in it in different individuals ,
and it is the colour of this that produces the colour of the eye , in the popular sense of the phrase . There is usually a remarkable correspondence in this point between the skin , the hair and the iris . A light complexion and hair are accompanied with blue , grey or lighter colours of the iris , but a dark skin and black hair with a
dark brown iris approaching to black . Under the choroides is found a third membrane of tKe eye-balls , called the retina , which is formed by the expansion of the optic nerve , and forms the
immediate object of vision . It is of a yellowish grey colour , and so extremely tender as to be lacerated by the slightest touch . Its outer surface is unconnected with the choroides and
the inner surface is expanded on the vitreous humour , but not connected with it . On the inside of the retina are seen branches of an artery and vein , which pass through the centre
* of the optic nerve . The part of the i rye at which the optic nerve enters is i nsensible , and hence physiologists have e xplained the reason why the optic 11 . erve is inserted . out of the axis of the
eje ; as otherwise the axis of vision w * ott ] d have fallen on an insensible part of the retina . 1 vitreous humour occupies the gr « atest part of the globular substance oft he eye . It consists of clear water con tainedin a cellular substance , which
is s <) perfectly transparent as to resemble pure glass , whence it derives its name . The cellular substance is condens ed on the surface into a smooth membrane , which is marked in front by a circular series of black radiated
lines , under these a circular canal runs . The crystalline humour or lens , so called from its transparency and shape , is imbedded in the front of the vitreous humour , in magnitude it is the sizie of a pea , but mote flattened in tf *
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shape . It is of a waxy consistence softer externally , and growing o ? a ' dually firmer towards the centre . This lens is contained in a capsule , which
may be split into two parts , and with this capsule it has no apparent connexion . The opaque state of this body constitutes the disease denominated a cataract .
The aqueous humour is a small quantity of transparent water placed immediately behind the cornea , and occupying" the space between that membrane and the crystalline lens : if by any accident this fluid is let out
it is very readily re-produced . In the midst of the space occupied b y this humour the iris is found , and it divides the space into two portions , called the anterior and posterior chambers of the eye , which communicate by means of the pupil .
Of * the eye-lids and lacrymal apparatus . The eye-ball is covered by two moveable curtains , called eye-lids . To keep these uniformly expanded , and to prevent them from forming wrinkles , each of them contains a thin portion of cartilage , adapted in figure to the
convexity of the globe : and in order to provide still farther for the greatest possible facility of motion , the eyelids are lined by a smooth and polished membrane , and the globe of the eye is covered by the same membrane ,
on its anterior part : this is denominated the conjunctiva , as it serves to connect the front of the eye-ball to the eye-lids . The eye-lids are opened by a muscle lifting up the upper lid , which is in a state of constant action
as long as our eyes are open , and they are closed by another muscle . The cilia , or eye-lashes are two rows of strong and curved hairs implanted in the opposite edges of the two eye-lids , and well adapted for protecting the
eye from dust and other foreign bodies . The hairy prominences above the eye-lids are called supercilia or eye-brows , these are very moveable , and serve as a protection to the eye * . They are much concerned in expressing the passions of the countenance .
To facilitate the motions of the eyelids and eye-balls on each , other , surface of the conjunctiva is continually moistened by a watery and mucilaginous fluid poured out by the arteries of that part . The incrustations of the mucilage in the night would , like glue , fasten the eye-lid *
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102 Natural Theology . No . II . —The Eye .
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), Feb. 2, 1815, page 102, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct1757/page/38/
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