• The two lacrimal canaliculi join to form a lacrimal sac.
• The tears flow into the superior and inferior canaliculi,
then to the lacrimal sac, and through the nasolacrimal
duct into the nasal cavity.
• The function of tears is to bathe the front of the eye,
washing away any dust, grit and microorganisms.
• Lysozyme destroys microorganisms present on the
front of the eyeball.
• In emotional states the secretion of tears may be
increased and if the nasolacrimal duct cannot carry
them all into the nasal cavity, they overflow.ACCoMModAtion
• accommodation is the reflex mechanism by which
focus of the lens is adjusted to different distances.
The least distance of distinct vision for normal eye is
25 cm and the maximum distance is infinity.
• accommodation is done by altering the shape of the
lens.
• Distance accommodation occurs when the ciliary
muscles are relaxed and the suspensory ligaments are
stretched resulting in reduced convexity of the lens and
increased focal length.
• Near accommodation occurs when the ciliary
muscles contract, relaxing the ligaments, causing lens
to return to its original convex form, thereby, reducing
focal length.VisuAL pErCEption
• Photoreceptors, rods and cones contain photopigments
composed of a protein called opsin, binding to a
chromophore.• Chromophore is the actual light-sensitive part of
photopigment, and is derived from vitamin a. It is an
aldehyde of vitamin a called retinal. It absorbs light
and transmits signal to cell through signal transduction
pathway.• Rhodopsin or visual purple present in the rods is
sensitive to low light levels and does not differentiate
colours. It is a combination of a protein scotopsin
and carotenoid pigment retinal or retinene which is
11-cis retinal form.
• On exposure to light, cis-retinal changes to all trans-
retinal, form which detaches from scotopsin. This
is because the 3-dimensional orientation of the
reactive sites of the all-trans-retinal no longer fit into
the orientation of the reactive sites on the protein
scotopsin.
• rhodopsin is formed again, when the all-trans-retinal
is converted to 11-cis-retinal catalyzed by retinal
isomerase.
• Once the 11-cis-retinal is formed, it automatically
combines with scotopsin to reform rhodopsin, which
then remains stable until its decomposition is again
triggered by absorption of light energy.
• Iodopsin is present in three types of cones, depending
on wavelength of light they absorb.