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1. The Haversian system consists of a central canal containing blood vessels and a nerve, surrounded by concentric rings of bony matrix. Between them, scattered tiny spaces called lacunae are filled with bone cells and connected by canaliculi to one another and the central canal. Through these canals the cells are nourished and kept alive.

2. The kidney, a very important organ, has the ability to secrete substances selectively which enables it to maintain proper composition of the blood and other body fluids. Some metabolic end products are injurious if allowed to accumulate.

3. Sex-linked genes explain red-green color blindness in humans. If a woman heterozygous for color blindness marries a normal-visioned man, half of her sons will be color blind. Her normal sons will show no trace of anomaly and will never transmit it to their children. All of her daughters will have normal vision. However, half of these will be heterozygous for the defect and can have color-blind sons. The homozygous daughters will never pass the trait on to either sons or daughters.

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