Structure Of The

The primary visual receptive tissue is the retina, a thin sheet of neurons with the consistency of wet cellophane. The position of the retina within the eyeball is somewhat analogous to the position of film in a camera (Fig. 1). Because light must enter the eye through the pupillary opening, the image that falls on the retina is reversed and inverted.

One class of retinal neurons, the photoreceptor cell, is primarily responsible for specific absorption of photons, and paradoxically these cells occupy the retinal layer furthermost from the incoming light (Fig. 2). In general, light must first penetrate through all retinal layers before encountering molecules of light-absorbing photopigment located in the outer segments of photoreceptors. As one might expect, this arrangement results in some distortion of the visual image. Regional specialization in the arrangement of retinal layers compensates for this distortion. The portion of the retina in line with the visual axis (a theoretical line drawn horizontally through the center of the lens) is modified so that all cellular layers of the retina are displaced radially except for the outer segments of photoreceptors. Even blood vessels are absent in this region. This allows incident light to project more directly on the photopigment-rich regions with minimal distortion. The cellular displacement results in a depression (or fovealpit) about 1.5 mm in diameter in the retinal surface. The fovea and the area immediately surrounding it are collectively referred to as the macula. The fovea is responsible for central, high-acuity vision. With functional loss of the fovea, an individual is considered legally sclera choroid retina blood vessels fovea in macula optic disk optic nerve sclera choroid retina blood vessels fovea in macula optic disk optic nerve

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Psychology Of Weight Loss And Management

Psychology Of Weight Loss And Management

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