In the growth of the individual embryo, as well as in evolutionary history, the brain develops roughly from the base of the skull up and outward. Indeed, in strictly biological terms, these structures can claim priority over the cerebral cortex. But underneath this layer reside many other specialized structures that are essential for movement, consciousness, sexuality, the action of our five senses, and moreвЂ”all equally valuable to human existence.

There are two types of cells or nerves that are used. The part of the brain responsible for vision is dependent on the receptors. This system is a fairly old part of the brain and one that humans share with many other vertebrates in reptiles, it is known as the rhinencephalon, or 'smell-brain,' because it reacts.

Thus, the left brain controls movement of the right side of the body, and the right brain controls movement of the left side.In addition to being the major site of crossover for nerve tracts running to and from the brain, the medulla is the seat of several pairs of nerves for organs of the chest and abdomen, for movements of the shoulder and head, for swallowing, salivation, and taste, and for hearing and equilibrium.At the top of the brainstem is the ponsвЂ”literally, a bridgeвЂ”between the lower brainstem and the midbrain. Through the medulla, at the lower end of the brainstem, pass all the nerves running between the spinal cord and the brain in the pyramids of the medulla, many of these nerve tracts for motor signals cross over from one side of the body to the other. This region is also an important junction for the control of deliberate movement. The brainstem, at the top of the spinal cord, controls breathing, the beating of the heart, and the diameter of blood vessels. The hindbrain contains several structures that regulate autonomic functions, which are essential to survival and not under our conscious control. Rods are wide and tall and are.

(Only the sense of smell sends signals directly to the cortex, bypassing the thalamus.) Sensations of pain, temperature, and pressure are also relayed through the thalamus, as are the nerve impulses from the cerebral hemispheres that initiate voluntary movement.The hypothalamus, despite its relatively small size (roughly that of a thumbnail), controls a number of drives essential for the functioning of a wide-ranging omnivorous social mammal. The masses contain nerve cell bodies that sort information from four of the sensesвЂ”sight, hearing, taste, and touchвЂ”and relay it to the cerebral cortex. The thalamus consists of two oval masses, each embedded in a cerebral hemisphere, that are joined by a bridge. Nerves in the midbrain also control some movements of the eyeball, pupil, and lens and reflexes of the eyes, head, and trunk.Deep in the core area of the brain, just above the top of the brainstem, are structures that have a great deal to do with perception, movement, and the body's vital functions. Together with nerves of the medulla, nerves from the pons also control breathing and the body's sense of equilibrium.What had been the middle bulge in the neural tube develops into the midbrain, which functions mainly as a relay center for sensory and motor nerve impulses between the pons and spinal cord and the thalamus and cerebral cortex. In addition, nerve fibers running through the pons relay sensations of touch from the spinal cord to the upper brain centers.Many nerves for the face and head have their origin in the pons, and these nerves regulate some movements of the eyeball, facial expression, salivation, and taste.

Physical signs of fear or excitement, such as a racing heartbeat, shallow breathing, and perhaps a clenched 'gut feeling,' all originate here.Also in the hypothalamus are neurons that monitor body temperature at the surface through nerve endings in the skin, and other neurons that monitor the blood flowing through this part of the brain itself, as an indicator of core body temperature. When strong feelings (rage, fear, pleasure, excitement) are generated in the mind, whether by external stimuli or by the action of thoughts, the cerebral cortex transmits impulses to the hypothalamus the hypothalamus may then send signals for physiological changes through the autonomic nervous system and through the release of hormones from the pituitary. It is the hypothalamus that first detects crucial changes in the body and responds by stimulating various glands and organs to release hormones.The hypothalamus is also the brain's intermediary for translating emotion into physical response. Thus it controls the rate of the heart, the passage of food through the alimentary canal, and contraction of the bladder.The hypothalamus is the main point of interaction for the body's two physical control systems: the nervous system, which transmits information in the form of minute electrical impulses, and the endocrine system, which brings about changes of state through the release of chemical factors.

The sensations that we interpret as hunger arise partly from a degree of emptiness in the stomach and partly from a drop in the level of two substances: glucose circulating in the blood and a hormone that the intestine produces shortly after the intake of food. In opposite conditions, when the body's temperature falls below the (rather narrow) ideal range, a portion of the hypothalamus directs the contraction of blood vessels, slows the rate of heat loss, and causes the onset of shivering (which produces a small amount of heat).The hypothalamus is the control center for the stimuli that underlie eating and drinking. Through its neurons associated with the sweat glands of the skin, the hypothalamus can also promote heat loss by increasing the rate of perspiration.

The pituitary gland also produces several hormones with more general effects: human growth hormone, melanocyte-stimulating hormone (which plays a role in the pigmentation of skin), and dopamine, which inhibits the release of prolactin but is better known as a neurotransmitter (see Chapter 5).The pineal gland produces melatonin, the hormone associated with skin pigmentation. The pituitary responds to signals from the hypothalamus by producing an array of hormones, many of which regulate the activities of other glands: thyroid-stimulating hormone, adrenocorticotropic hormone (which stimulates an outpouring of epinephrine in response to stress), prolactin (involved in the production of milk), and the sex hormones follicle-stimulating hormone and luteinizing hormone, which promote the development of eggs and sperm and regulate the timing of ovulation. Because of its unique position as a midpost between thought and feeling and between conscious act and autonomic function, a thorough understanding of its workings should tell us much about the earliest history and development of the human animal.The pituitary and the pineal glands function in close association with the hypothalamus. In all, the hypothalamus is a richly complex cubic centimeter of vital connections, which will continue to reward close study for some time to come. For one thing, it forms part of the reticular activating system, the physical basis for that hard-to-define state known as consciousness (about which more later) for another, electrical stimulation of a portion of the hypothalamus has been shown to induce sleep in experimental animals, although the mechanism by which this works is not yet known. In experimental animals, damage to this portion of the brain is associated with continued excessive eating, eventually leading to obesity.In addition to these numerous functions, there is evidence that the hypothalamus plays a role in the induction of sleep.

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.

What Part Of The Brain Controls Vision