ies significantly over a 24-hour cycle, from low levels during the day to a peak at night, and the pineal gland has been called a “ third eye” because it is controlled by neurons sensitive to light, which originate in the retina of each eye and end in the hypothalamus. In animals with a clear-cut breeding season, the pineal gland is a link between the shifting hours of daylight and the hormonal responses of the hypothalamus, which in turn guide reproductive functions. In humans, who can conceive and give birth throughout the year, the pineal gland plays no known role in reproduction, although there is evidence that melatonin has a share in regulating ovulation.

THE “LITTLE BRAIN” AT THE BACK OF THE HEAD

While autonomic and endocrine functions are being maintained by structures deep inside the brain, another specialized area is sorting and processing the signals required to maintain balance and posture and to carry out coordinated movement. The cerebellum (the term in Latin means “little brain”) is actually a derived form of the hindbrain—as suggested by its position at the back of the head, partly tucked under the cerebral hemispheres. In humans, with our almost unlimited repertoire of movement, the cerebellum is accordingly large; in fact, it is the second-largest portion of the brain, exceeded only by the cerebral cortex. Its great surface area is accommodated within the skull by elaborate folding, which gives it an irregular, pleated look. In relative terms, the cerebellum is actually largest in the brain of birds, where it is responsible for the constant streams of information between brain and body that are required for flight.

In humans, the cerebellum relays impulses for movement from the motor area of the cerebral cortex to the spinal cord; from there, they pass to their designated muscle groups. At the same time, the cerebellum receives impulses from the muscles and joints that are being activated and in some sense compares them with the instructions issued from the motor cortex, so that adjustments can be made (this time by way of the thalamus). The cerebellum thus is neither the sole initiator of movement nor a simple link in the chain of nerve impulses, but a site for the rerouting and in some cases refining of instructions for movement. There is evidence, too, that the cerebellum can



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