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The Mammalian eye.

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Introduction

Sumudu D. Lankatilake U6B Biology The Mammalian eye The eyes are spherical structures held in the bony sockets of the skull called the orbits. The rectus and oblique muscles control the eye movement. The wall of the eye is composed of three distinct layers, which are the sclera, choroid, and retina. The biconvex crystalline lens is held inside the eye behind the pupil by suspesory ligaments, which in turn are attached to the ciliary body (smooth muscle). Aqueous and vitreous humour maintains the shape of the eye. The lachrymal glands secrete a bactericidal enzyme, which protects the eye from abrasion and infection. ...read more.

Middle

Their functioning depends on the light stimulus being detected by the photosensitive pigment: rhodopsin in the rods and iodopsin in the cones. Rods Rods are distributed evenly around the retina but are absent in the fovea. They are sensitive to different intensities of light and are involved with vision at low light intensities. The rods contain rhodopsin in its outer segment in flattened vesicles called lamellae. The inner segment contains large numbers of mitochondria, polysomes and a nucleus. The mitochondria contain ATP for the resynthesis of the rhodopsin. The polysomes are for the synthesis of proteins and the production of visual pigment. ...read more.

Conclusion

Cones Cones differ form rods in the following ways * The outer segment is cone shaped * There are fewer membranous vesicles and they are formed from infoldings of the outer membrane. * They contain the visual pigment iodopsin. There are fewer cones than rods, and are concentrated around or at the fovea. They are les sensitive to light but more sensitive to the wavelength of light. There are three different types for red blue and green light, involving different forms of iodopsin. According to the trichromatic theory of colour vision, different colours are perceived according to the degree of stimulation of each type of cone by light reflected objects. Deficiency of one or more one types produces colour blindness. The red-green colour blindness is often common which is due to a lack of red cones. ...read more.

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