. Human physiology. nea. «, part of the sclerotic thrown back ; i, ciliary muscle ; c, iris ; t, one of theciliary nerves ; f, blood-vessel. ball. It forms the prominent spherical surface before mentioned,and is clear and transparent. It is not supplied with blood-vessels. The choroid coat is a dark-brown membrane lying within andagainst the sclerotic. It consists of a thickly set network of blood-vessels, supported by connective tissue, and loaded with cells con-taining a dark pigment or colouring matter. One use of this coat,as we shall presently learn, is to darken the chamber of the eye,an
. Human physiology. nea. «, part of the sclerotic thrown back ; i, ciliary muscle ; c, iris ; t, one of theciliary nerves ; f, blood-vessel. ball. It forms the prominent spherical surface before mentioned,and is clear and transparent. It is not supplied with blood-vessels. The choroid coat is a dark-brown membrane lying within andagainst the sclerotic. It consists of a thickly set network of blood-vessels, supported by connective tissue, and loaded with cells con-taining a dark pigment or colouring matter. One use of this coat,as we shall presently learn, is to darken the chamber of the eye,and thus prevent the reflection of light. Just before the choroidcoat reaches the edge of the cornea, it becomes modified, being 206 ELEMENTARY PHYSIOLOGY raised into a number of ridges which constitute the ciliary pro-cesses. The iris (Lat. a window) is the circular curtain, seen throughthe cornea, which gives the colour to the eye. Its outer borderis continuous with the choroid coat, which, like the sclerotic, does. Fig. 190.—View of the Human Eye, divided horizontally throughthe middle. x, conjunctiva; 2, cornea J 3, sclerotic ; 4, sheath of the optic nerve; 5, choroid ;6, ciliary processes ; 7, iris ; 8, pupil; 9, retina ; 10, anterior limit of the retina;11, crystalline lens; 12, suspensory ligament; 13, ciliary muscle; 14, aqueouschamber; 15, vitreous chamber; 16, yellow spot ; 17, blind spot. not extend over the front of the ball. It is perforated in itscentre by a circular aperture called the pupil. The iris is a con-tractile membrane. It is provided with unstriped muscular fibres,some of which are arranged in a ring around the pupil, while theothers are radiating. When the circular fibres contract, the pupilis made smaller; when the radiating fibres contract the pupil is THE EYE AND VISION 207 dilated. By these means the diameter of the pupil is made to varyfrom Jrd to ^th of an inch, and the quantity of light admittedinto the eyeball is regulated. Theouter rim of the
Size: 1476px × 1693px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookpublisherlondon, booksubject
