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

References in classic literature ?
I've made such quantities it would be hard to choose which I'd have," said Laurie, lying flat and throwing cones at the squirrel who had betrayed him.
In other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance.
A bird sits on the next bough, life-everlasting grows under the table, and blackberry vines run round its legs; pine cones, chestnut burs, and strawberry leaves are strewn about.
of talk and laughter--and, if you will, the Wurzburger in the tall glass cones that bend to your lips as a ripe cherry sways on its branch to the beak of a robber jay.
Joseph, followed grinning, in the Collector's rear, and bearing two handsome nosegays of flowers, which the monster had actually had the gallantry to purchase in Covent Garden Market that morning--they were not as big as the haystacks which ladies carry about with them now-a-days, in cones of filigree paper; but the young women were delighted with the gift, as Joseph presented one to each, with an exceedingly solemn bow.
Not the wondrous cistern in the whale's huge head; not the prodigy of his unhinged lower jaw; not the miracle of his symmetrical tail; none of these would so surprise you, as half a glimpse of that unaccountable cone, -- longer than a Kentuckian is tall, nigh a foot in diameter at the base, and jet-black as Yojo, the ebony idol of Queequeg.
One may picture, too, the sudden shifting of the attention, the swiftly spreading coils and bellyings of that blackness advancing headlong, towering heavenward, turning the twi- light to a palpable darkness, a strange and horrible antagonist of vapour striding upon its victims, men and horses near it seen dimly, running, shrieking, falling headlong, shouts of dismay, the guns suddenly abandoned, men choking and writhing on the ground, and the swift broadening-out of the opaque cone of smoke.
He threw a pine cone at a jovial squirrel, and he ran with chattering fear.
Dominic had been sitting motionless, like an inanimate black cone posed on the stern deck, near the rudder-head, with a small tassel fluttering on its sharp point, and for a time he preserved the immobility of his meditation.
As auxiliary to this scarcity of fuel, one of the large springs which abound in that country gushed out of the side of the ascent above, and, after creeping sluggishly along the level land, saturating the mossy covering of the rock with moisture, it swept around the base of the little cone that formed the pinnacle of the mountain, and, entering the canopy of smoke near one of the terminations of the terrace, found its way to the lake, not by dashing from rock to rock, but by the secret channels of the earth.
Raoul perceived, from a distance, the two little turrets, the dove-cote in the elms, and the flights of pigeons, which wheeled incessantly around that brick cone, seemingly without power to quit it, like the sweet memories which hover round a spirit at peace.
He is worshipped under the figure of a large stone pillar terminating at the summit in a cone or pyramid, whereby is denoted Fire.