Vision

Freddy SantiagoPeriod 2

2 Vision Anatomy & Physiology

We rely more on vision than on any other special senses. Our visual devices are contained in the

eyes, a complex structures that enables us to detect not only light but also detailed images. We will begin our discussion of these complex organs by consider-ing the accessory structures of the eye, which protect, make them smooth, and support.

Vision

Contents

Vision � � � � � � � � � � � � � � � 2

The Accessory Structures of the Eye � � � � � � � � � � � � 3

The Eye � � � � � � � � � � � � � � 4

Image Formation � � � � � 6

Rods and cones � � � � � � � 7

Anatomy & Physiology Vision 3

The Accessory Structures of the Eye

The accessory structures of the eye include the eyelids and associated exocrine glands, or a cell

or group of cells that secrets a specific substance, and the superficial epithelium, a thin protective layer of the eye, structures associated with the production, secretion, and removal of tears and the eye muscles.

The eyelids are a continuation of the skin. They act like windshield wipers: Their blinking movements keep the surface of the eye to run smoothly and free from dust and tiny fragments that has been broken into pieces. They can also close firmly to protect the delicate surface of the eye. The eyelashes are strongly constructed hairs that help prevent foreign particles such as dirt and insects from contacting the surface of the eye.

A constant flow of tears keep the surface of the eye-ball moist and clean. Tears reduce friction, remove debris, and prevent bacterial infections. Blinking, sweeps tears across the surface of the eye to the corner at each side of the eye. Two tiny openings di-rect the tears into the lacrimal canals, passageways through which tears drain. From this sac, the tears go through a tiny canal in the nasal cavity which is how your nose gets runny when your eyes tear up. You can move your eyes with the aid of six eye muscles that originate on the surface of the orbit and control the eyes position.

4 Vision Anatomy & Physiology

The EyeThe eyes are advanced visual instruments—has

more uses in many different ways and adaptable than most expensive cameras, yet compact and dura-ble. Each eye is roughly spherical, with a diameter of nearly 2.5 cm (1 in.) and weighs around 8g (0.28 oz) with the eyeball being hollow.

Eye color is determined by the number of melano-

cytes, cells that produce pigments (a substance that gives something its color), in the iris and the presence of melanin granules in the pigmented thin layer of tis-sue on the back surface of the iris.

The retina is the inner most layer of the eye. It con-sists of pigmented layer called the pigmented part and a thick inner layer called the neural part. The pig-mented part absorbs light after it passes through the neural part. The neural part contains the photorecep-tors, cell that respond to light, supporting cells ,neu-

Anatomy & Physiology Vision 5

rons that perform preliminary processing, integration of visual information, and blood vessels supplying tissues that line the posterior cavity.

The retina contains several layers of cells. The outer most layer, closest to the wall of the pigmented part of the retina, contains the photoreceptors, the cells that detect light. The eye has two types of photorecep-tors: rods and cones.

Rods do not discriminate among colors of light. These very light-sensitive receptors enable us to see in dimly lit rooms, at twilight, or in pale moon light.

Cones provide us with color vision. Three types of cones are present, and their response in various com-binations provides the perception of different colors. Cones give us sharper, clearer images, but require brighter light than do rods. When you watch a sunset, you can notice you vision shifting from cone-based vision (a clear image in full color) to rod-based vision (less distinct image in black and white).

The optic disc, a circular region, has no photorecep-tors or other retinal structures. Because light striking goes unnoticed, it’s commonly called the blind spot. You do not notice a blank spot in your visual field because voluntary eye movements keep the visual im-age moving and allow your brains to fill in the missing information.

The Sectional Anatomy of the Eye� (a) This sagit-tal section bisects the left eye. (b) This horrizontal section reveals the three layers, or tunics, of the right eye. (c) This horizontal section of the right eye shows various landmarks and features, including the path light takes (the “visual axis”).

6 Vision Anatomy & Physiology

Image Formation

The image formation of an object reaching the ret-ina is a miniature image of the original, but up-

side down and backwards. The brain compensates for both aspects of image reversal without our conscious awareness, we don’t even notice.

Retinal Organization� (a) The cellular organization of the retina is shown in a drawing and a micrograph. Note that the photoreceptors are closer to the choroid than to the posterior cavity. (b) This diagrammatic section passes through the optic disc. (c) This photo-graph shows the retina as seen through the pupil of the right eye. Image Formation� Light from each portion of an

objects is focused on a different part of the retina. The resulting image arrives (a) upside down and (b) back-ward.

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Rods and conesWe have three types of cones: blue cones, green

cones, and red cones. Each type of cone contains pigments sensitive to blue, green, or red wavelengths of light.

Persons unable to distinguish certain colors have

some form of color blindness. The standard test for color vision involves picking number or letters out of a complex image. Color Blindness occurs because one or more classes of cones are absent or nonfunctional. 10% of men have some color blindness, whereas the incidence among women is less than one percent. To-tal color blindness is extremely rare; only 1 person in 30,000 has no cone pigments of any kind.

The Structure of Rods and Cones� (a) The names rods and cones refer to the shapes of the photoreceptor’s outer segment. Their membrane discs contain the visual pigments. (b) Visual pig-ments are derived from rhodopsin molecules within the membrane of a disc.