The retina is actually part of the brain that is isolated to serve as a transducer for the conversion of light into neuronal signals.
Note that up until now much of the above paragraph could apply to octopuses, mollusks, worms, insects and things more primitive; anything with a more concentrated nervous system and better eyes than say a jellyfish.
However, the following applies to mammals generally and birds (in modified form): The retina in these more complex animals sends fibers (the optic nerve) to the lateral geniculate nucleus, to the primary and secondary visual cortex of the brain.
[2] The human visual system perceives visible light in the range of wavelengths between 370 and 730 nanometers (0.00000037 to 0.00000073 meters) of the electromagnetic spectrum.
[3] Hearing, or auditory perception, is the ability to perceive sound by detecting vibrations,[4] changes in the pressure of the surrounding medium through time, through an organ such as the ear.
[8] Olfaction occurs when odorants bind to specific sites on olfactory receptors located in the nasal cavity.
[9] Glomeruli aggregate signals from these receptors and transmit them to the olfactory bulb, where the sensory input will start to interact with parts of the brain responsible for smell identification, memory, and emotion.
A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter.
The mucus overlying the epithelium contains mucopolysaccharides, salts, enzymes, and antibodies (these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the brain).
This mucus acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes.
Taste, along with smell (olfaction) and trigeminal nerve stimulation (registering texture, pain, and temperature), determines flavors of food or other substances.
[25] Among humans, taste perception begins to fade around 50 years of age because of loss of tongue papillae and a general decrease in saliva production.