Two theories together account for color vision. in stage one, the helmholtz trichromatic theory shows that the eye contains while in stage two, hering's theory accounts for the nervous system's which are activated by one color light, and inhibited by the opposite color light.

b - the respond is innate and varies over time.

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thrombocytopenia, which is a deficiency in blood platelets, can cause bleeding and bruising in the dermis similar to senile purpura. other possible causes include: exposure to ultraviolet rays and aging skin are common causes of senile purpura. senile purpura may also be a sign of collagen loss in the skin and bones.