#89 Structure and function of the eye, rods and cones

You need to be able to label parts of the eye on diagrams. 

• The eyebrow stops sweat running down into the eye. 
• Eyelashes help to stop dust blowing on to the eye. 
• Eyelids can close automatically (blinking is a reflex) to prevent dust and other particles getting ton to the surface of the cornea. 
• Blinking also helps to keep the surface moist by moving liquid secretions (tears) over the exposed surface. Tears also contain enzymes that have an antibacterial function. 

* Try this 

Trace or copy both diagrams of the eyes. Practice adding the labels. [8 marks] 

Distinguishing between rods and cones 

Rods and cones are light-sensitive cells in the retina. When stimulated they generate electrical impulses, which pass to the brain along the optic nerve. 

The normal retina has rods that see only black, white, and shades of grey and tones and three forms of color cones, red, green, and blue. 

	Function	Distribution	Comments
Rods	Sensitive to low light intensity. Detect shades of grey	Found throught the retina, but none in the centrer of the fovea or in the blind spot	Provide us with night vision, when we can recognise shapes but not colours
Cones	Sensitive only to high light intensity. Detect colour (don’t operate in poor light)	Concentrated in the fovea	There are three types, sensitive to red, green and blue light

More photos of rods and cones.