Cones


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Cones

 

in man and vertebrates, cone-shaped photoreceptors of the eye that function in the perception of daylight and provide color vision. They are located, together with the rods, in the outer layer of the retina.

The cones are made up of an external segment, an internal segment, a connective fiber, a nucleus-containing portion, and an internal fiber that ends in a thickening in which a synaptic juncture is established with the bipolar and horizontal nerve cells of the retina. The ultrastructure of the cone indicates that the external segment is a derivative of cilia. It is constructed of numerous membranous disks containing visual pigment. The internal segment contains mitochondria and a fat drop. In many vertebrates the internal segment also contains a contractile element called the myoid. The external and internal segments are joined by a delicate structure that consists of nine pairs of double threads distributed along the periphery; the central pair of threads, characteristic of movable cilia, is absent.

Cones predominate over rods in the retinas of diurnal animals (for example, the suslik has only rods). In man, cones predominate at the center of the retina and rods predominate along the edges. The central depression, or fovea, contains only cones.

O. G. STROEVA

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