1. Referred pain results from the fact that sensory neurons from different organs converge on the same CNS interneurons, making it difficult for the brain to identify the source of stimulation. In skotopic vision, hundreds of rod cells converge on one bipolar cell and many bipolar cells converge on each ganglion cell. In dim light, stimulation of one rod cell may not produce a sensation, but spatial summation allows many rod cells to produce an additive effect on a ganglion cell.

2. Hair receptors would detect the louse; tactile (Meissner) corpuscles are involved in reading Braille; and lamellated (pacinian) corpuscles allow one to feel a pulse with the fingertips.

3. When the ciliary muscle contracts, it narrows the diameter of the ciliary body and reduces tension on the suspensory ligament of the lens. The lens is then less stretched and relaxes into a more convex shape.

4. In both cases, it becomes difficult to focus on objects close to the eye. The underlying cause, however, is different. Presbyopia results from stiffening of the lens with age so that it cannot accommodate as well, whereas hyperopia results from an eyeball that is too short for the retina to lie on the focal plane of the lens.

5. Like pattern baldness, color blindness results from an X-linked recessive allele. Men are more likely than women to be color blind because they have only one X chromosome. If that chromosome has the allele for color blindness, the trait will be expressed in the male. In a female, however, it will not be expressed unless her other X chromosome also happens to carry that allele, which would only happen in the relatively improbable event that both of her parents passed the trait to her.

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