Visual optics and sight testing / by Lionel Laurance.

  • Laurance, Lionel, 1856?-1936.
Date:
1912
Catalogue details

Licence: In copyright

Credit: Visual optics and sight testing / by Lionel Laurance. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    The First Coat consists of the sclerotic and cornea, their junction being termed the sclero-corneal margin. The Sclerotic is a tough, opaque, fibrous membrane which surrounds about five-sixths of the globe. It is about 1 mm. thick at the back, thins slightly towards the equator, to again become somewhat thicker at its junction with the cornea. The sclerotic may be regarded as the wall or protective envelope of the eye, its functions being to preserve the contents from injury and to maintain its shape. Its colour is milk-white, but, if thin, has a bluish tinge, and if pigmentation is heavy, as in negroes, it is yellowish. The Cornea projects from the front of the sclerotic, and covers the remaining one-sixth of the globe; its tissue is elastic and transparent, and it may be regarded as the window of the eye, through which the light is admitted. The cornea, with the aqueous, forms the first and principal refracting medium in the dioptric system and the two resemble a segment of a sphere of transparent celluloid. The diameter of the cornea is 12 or 13 mm., and its average thickness 1 mm., but it is rather thinner at the apex than at the scleral margin. The radius of curvature of the anterior surface is about 8 mm., that of its posterior surface about 6-5 mm., so that the cornea forms a concave meniscus whose index of refraction is taken to be 1-33. The cornea of an infant does not differ much in general dimensions from that of an adult. The cornea has three layers— 1. The outer, or epithelial, which is continuous with the epithelial layer of the conjunctiva. 2. The middle, or true corneal, which occupies the greater part of the corneal thickness. 3. The internal, or membrane of Descemet, which is a highly elastic layer lined internally by cells continuous with those on the front surface of the iris. An incomplete transparent membrane (Bowman's) lies between Nos. 1 and 2. The cornea has no bloodvessels in its substance, but is nourished by lymph flowing between the laminae of the middle layer. It is richly supplied with fine nerve endings, which render it highly sensitive to touch and foreign bodies. The Second Coat consists of the choroid proper, the ciliary body, and the iris. The Choroid is composed of connective tissue, bloodvessels, and dark brown pigment cells ; the processes of the latter, in the presence of light, elongate and contract, and are evidently connected with vision since they exist, to a greater or lesser degree, in all animals which have light- perceiving organs. The chief function of the choroid is probably nvirUtve,