Sensory Organs Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 18, 2011

Vascular Tunic (middle layer)

The vascular tunic has three parts. The choroid is a thin, highly vascular layer that supplies nutrients and oxygen to the eye and absorbs light, preventing it from being reflected. The ciliary body is the thickened anterior portion of the vascular tunic. It contains smooth muscle fibers that regulate the shape of the lens. The iris forms the most anterior portion of the vascular tunic and consists of pigment (that gives the eye color) and smooth muscle fibers arranged in a circular and radial pattern that regulate the diameter of the pupil, which is the opening in the center of the iris.

Internal Tunic (inner layer, or retina)

The receptor component of the eye contains two types of photoreceptors. Cones (approximately 7 million cones per eye) function at high light intensities and are responsible for daytime color vision and acuity (sharpness). Rods (approximately 100 million per eye) function at low light intensities and are responsible for night (black-and-white) vision. The retina also contains bipolar cells, which synapse with the rods and cones, and ganglion cells, which synapse with the bipolar cells. The axons of the ganglion cells course along the retina to the optic disc and form the optic nerve (CN II). The fovea centralis is a shallow pit at the back of the retina that contains only cones. It is the area of keenest vision. Surrounding the fovea centralis is the macula lutea, which also has an abundance of cones.


The lens is a transparent, biconvex structure composed of tightly arranged proteins. It is enclosed in a lens capsule and held in place by the suspensory ligament that attaches to the ciliary body. The lens focuses light rays for near and far vision.

Cavities of the Eye

The interior of the eye is separated by the lens into an anterior cavity and a posterior cavity (vitreous chamber). The anterior cavity is partially subdivided by the iris into an anterior and a posterior chamber. The anterior cavity contains a watery fluid called aqueous humor. The posterior cavity contains a transparent jellylike substance called vitreous humor.


The field of vision is what a person visually perceives. There are three visual fields, the macular field, the area of keenest vision, the binocular field, the portion viewed by both eyes, but not keenly focused on, and the monocular field, that area viewed by one eye and not shared by the other.

The neural pathway for vision consists of the light rays striking the photoreceptors in the retina, which causes the transmission of nerve impulses along the optic nerve to the optic chiasma. The optic tract, a continuation of optic nerve fibers from the optic chiasma, carries the impulses to the occipital cerebral lobes where vision occurs.

For an image to be focused on the retina, the more distant the object, the flatter must be the lens. Adjustments in lens shape, accomplished by the ciliary muscles in the ciliary body, are called accommodation. When these smooth muscles contract, the fibers within the suspensory ligament slacken, causing the lens to thicken and become more convex.

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