The organ of vision, strictly speaking, consists only of the ball or globe of the eye ; but connected with the eyeball externally are muscles, nerves, and blood-vessels, elsewhere described, as well as other parts specially destined for its protection, and known as the appendages of the eye (tutamina oculi), of which an account will first be given.
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye.
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The eyelids (palpebrae) are moveable portions of integument, strengthened toward their margins by a thin lamina of dense fibrous tissue. A mucous membrane lines their inner surface, and is reflected thence in the form of a pellucid covering on the surface of the eyeball. This is named the conjunctival membrane or conjunctiva.
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The globe or ball of the eye is a composite structure of spheroidal form, placed in the fore part of the orbital cavity, and receiving the thick stem of the optic nerve behind. The recti and oblique muscles closely surround the greater part of the eyeball, and are capable of changing its position within certain limits: the lids, with the plica semilunaris and caruncle, are in contact with its covering of conjunctiva in front; and behind it is supported by a quantity of loose fat and connective tissue.
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The parts which constitute the lachrymal apparatus are the following, viz.: - The gland by which the tears are secreted, situated at the upper and outer side of the orbit, together with its excretory ducts; the two canals into which the fluid is received near the inner angle: and the sac with the nasal duct continued from it, through which the tears pass into the inferior meatus of the nose.
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